diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/anyof_model.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/anyof_model.mustache
index 97bc82c6e6b8..8c15d70b1954 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/anyof_model.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/anyof_model.mustache
@@ -81,7 +81,7 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
                     {{^isPrimitiveType}}
                     {{#isArray}}
                         List<?> list = (List<?>) value.getActualInstance();
-                        if (list.get(0) instanceof {{{items.dataType}}}) {
+                        if (!list.isEmpty() && list.get(0) instanceof {{{items.dataType}}}) {
                             JsonArray array = adapter{{#sanitizeDataType}}{{{dataType}}}{{/sanitizeDataType}}.toJsonTree(({{{dataType}}})value.getActualInstance()).getAsJsonArray();
                             elementAdapter.write(out, array);
                             return;
@@ -252,7 +252,7 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
         if (instance instanceof {{#isArray}}List<?>{{/isArray}}{{^isArray}}{{{dataType}}}{{/isArray}}) {
         {{#isArray}}
             List<?> list = (List<?>) instance;
-            if (list.get(0) instanceof {{{items.dataType}}}) {
+            if (!list.isEmpty() && list.get(0) instanceof {{{items.dataType}}}) {
                 super.setActualInstance(instance);
                 return;
             }
diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/oneof_model.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/oneof_model.mustache
index 8684087c08eb..a92376ba1653 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/oneof_model.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/oneof_model.mustache
@@ -94,7 +94,7 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
                     {{^isPrimitiveType}}
                     {{#isArray}}
                         List<?> list = (List<?>) value.getActualInstance();
-                        if (list.get(0) instanceof {{{items.dataType}}}) {
+                        if (!list.isEmpty() && list.get(0) instanceof {{{items.dataType}}}) {
                             JsonArray array = adapter{{#sanitizeDataType}}{{{dataType}}}{{/sanitizeDataType}}.toJsonTree(({{{dataType}}})value.getActualInstance()).getAsJsonArray();
                             elementAdapter.write(out, array);
                             return;
@@ -330,7 +330,7 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
         if (instance instanceof {{#isArray}}List<?>{{/isArray}}{{#isMap}}Map<?, ?>{{/isMap}}{{^isMap}}{{^isArray}}{{{dataType}}}{{/isArray}}{{/isMap}}) {
         {{#isArray}}
             List<?> list = (List<?>) instance;
-            if (list.get(0) instanceof {{{items.dataType}}}) {
+            if (!list.isEmpty() && list.get(0) instanceof {{{items.dataType}}}) {
                 super.setActualInstance(instance);
                 return;
             }
diff --git a/modules/openapi-generator/src/main/resources/kotlin-client/anyof_class.mustache b/modules/openapi-generator/src/main/resources/kotlin-client/anyof_class.mustache
index bf2a04d01d9b..213b2fc15645 100644
--- a/modules/openapi-generator/src/main/resources/kotlin-client/anyof_class.mustache
+++ b/modules/openapi-generator/src/main/resources/kotlin-client/anyof_class.mustache
@@ -110,7 +110,7 @@ import java.io.IOException
                     if (value.actualInstance is {{#isArray}}List<*>{{/isArray}}{{^isArray}}{{{dataType}}}{{/isArray}}) {
                     {{#isArray}}
                         val list = value.actualInstance as List<Any>
-                        if (list.get(0) is {{{items.dataType}}}) {
+                        if (!list.isEmpty() && list.get(0) is {{{items.dataType}}}) {
                             val array = adapter{{#sanitizeGeneric}}{{{dataType}}}{{/sanitizeGeneric}}.toJsonTree(value.actualInstance as {{{dataType}}}?).getAsJsonArray()
                             elementAdapter.write(out, array)
                             return
diff --git a/modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache b/modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache
index 156ed8d269a3..a6423598c0b9 100644
--- a/modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache
+++ b/modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache
@@ -110,7 +110,7 @@ import java.io.IOException
                     if (value.actualInstance is {{#isArray}}List<*>{{/isArray}}{{^isArray}}{{{dataType}}}{{/isArray}}) {
                     {{#isArray}}
                         val list = value.actualInstance as List<Any>
-                        if (list.get(0) is {{{items.dataType}}}) {
+                        if (!list.isEmpty() && list.get(0) is {{{items.dataType}}}) {
                             val array = adapter{{#sanitizeGeneric}}{{{dataType}}}{{/sanitizeGeneric}}.toJsonTree(value.actualInstance as {{{dataType}}}?).getAsJsonArray()
                             elementAdapter.write(out, array)
                             return
diff --git a/samples/client/others/java/okhttp-gson-oneOf-array/src/main/java/org/openapitools/client/model/MyExampleGet200Response.java b/samples/client/others/java/okhttp-gson-oneOf-array/src/main/java/org/openapitools/client/model/MyExampleGet200Response.java
index 3bbba5d18c55..fe8f99b297c2 100644
--- a/samples/client/others/java/okhttp-gson-oneOf-array/src/main/java/org/openapitools/client/model/MyExampleGet200Response.java
+++ b/samples/client/others/java/okhttp-gson-oneOf-array/src/main/java/org/openapitools/client/model/MyExampleGet200Response.java
@@ -89,7 +89,7 @@ public void write(JsonWriter out, MyExampleGet200Response value) throws IOExcept
                     // check if the actual instance is of the type `List<@Valid OneOf1>`
                     if (value.getActualInstance() instanceof List<?>) {
                         List<?> list = (List<?>) value.getActualInstance();
-                        if (list.get(0) instanceof OneOf1) {
+                        if (!list.isEmpty() && list.get(0) instanceof OneOf1) {
                             JsonArray array = adapterListOneOf1.toJsonTree((List<@Valid OneOf1>)value.getActualInstance()).getAsJsonArray();
                             elementAdapter.write(out, array);
                             return;
@@ -191,7 +191,7 @@ public Map<String, Class<?>> getSchemas() {
     public void setActualInstance(Object instance) {
         if (instance instanceof List<?>) {
             List<?> list = (List<?>) instance;
-            if (list.get(0) instanceof OneOf1) {
+            if (!list.isEmpty() && list.get(0) instanceof OneOf1) {
                 super.setActualInstance(instance);
                 return;
             }
diff --git a/samples/client/petstore/java/okhttp-gson-3.1/src/main/java/org/openapitools/client/model/RefToRefParameterAnyofRefToAnyofParameter.java b/samples/client/petstore/java/okhttp-gson-3.1/src/main/java/org/openapitools/client/model/RefToRefParameterAnyofRefToAnyofParameter.java
index 93b88a182ca2..8070f39f1b37 100644
--- a/samples/client/petstore/java/okhttp-gson-3.1/src/main/java/org/openapitools/client/model/RefToRefParameterAnyofRefToAnyofParameter.java
+++ b/samples/client/petstore/java/okhttp-gson-3.1/src/main/java/org/openapitools/client/model/RefToRefParameterAnyofRefToAnyofParameter.java
@@ -187,7 +187,7 @@ public void setActualInstance(Object instance) {
 
         if (instance instanceof List<?>) {
             List<?> list = (List<?>) instance;
-            if (list.get(0) instanceof String) {
+            if (!list.isEmpty() && list.get(0) instanceof String) {
                 super.setActualInstance(instance);
                 return;
             }
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ArrayAnyOf.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ArrayAnyOf.java
index 0c6bb5dca69d..f95661c16930 100644
--- a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ArrayAnyOf.java
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ArrayAnyOf.java
@@ -182,7 +182,7 @@ public void setActualInstance(Object instance) {
 
         if (instance instanceof List<?>) {
             List<?> list = (List<?>) instance;
-            if (list.get(0) instanceof String) {
+            if (!list.isEmpty() && list.get(0) instanceof String) {
                 super.setActualInstance(instance);
                 return;
             }
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ArrayOneOf.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ArrayOneOf.java
index de22a4895a35..7797c7c029af 100644
--- a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ArrayOneOf.java
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ArrayOneOf.java
@@ -187,7 +187,7 @@ public void setActualInstance(Object instance) {
 
         if (instance instanceof List<?>) {
             List<?> list = (List<?>) instance;
-            if (list.get(0) instanceof String) {
+            if (!list.isEmpty() && list.get(0) instanceof String) {
                 super.setActualInstance(instance);
                 return;
             }
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FakeAnyOfWIthSameErasureGet200Response.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FakeAnyOfWIthSameErasureGet200Response.java
index af553ff63059..63c07438a36f 100644
--- a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FakeAnyOfWIthSameErasureGet200Response.java
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FakeAnyOfWIthSameErasureGet200Response.java
@@ -157,7 +157,7 @@ public Map<String, Class<?>> getSchemas() {
     public void setActualInstance(Object instance) {
         if (instance instanceof List<?>) {
             List<?> list = (List<?>) instance;
-            if (list.get(0) instanceof String) {
+            if (!list.isEmpty() && list.get(0) instanceof String) {
                 super.setActualInstance(instance);
                 return;
             }
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FakeOneOfWIthSameErasureGet200Response.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FakeOneOfWIthSameErasureGet200Response.java
index 5c30a78aafd2..bc19f2ec8825 100644
--- a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FakeOneOfWIthSameErasureGet200Response.java
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FakeOneOfWIthSameErasureGet200Response.java
@@ -163,7 +163,7 @@ public Map<String, Class<?>> getSchemas() {
     public void setActualInstance(Object instance) {
         if (instance instanceof List<?>) {
             List<?> list = (List<?>) instance;
-            if (list.get(0) instanceof String) {
+            if (!list.isEmpty() && list.get(0) instanceof String) {
                 super.setActualInstance(instance);
                 return;
             }
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Value.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Value.java
index d047baaa90dd..174dc779d31e 100644
--- a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Value.java
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Value.java
@@ -86,7 +86,7 @@ public void write(JsonWriter out, Value value) throws IOException {
                     // check if the actual instance is of the type `List<Scalar>`
                     if (value.getActualInstance() instanceof List<?>) {
                         List<?> list = (List<?>) value.getActualInstance();
-                        if (list.get(0) instanceof Scalar) {
+                        if (!list.isEmpty() && list.get(0) instanceof Scalar) {
                             JsonArray array = adapterListScalar.toJsonTree((List<Scalar>)value.getActualInstance()).getAsJsonArray();
                             elementAdapter.write(out, array);
                             return;
@@ -187,7 +187,7 @@ public void setActualInstance(Object instance) {
 
         if (instance instanceof List<?>) {
             List<?> list = (List<?>) instance;
-            if (list.get(0) instanceof Scalar) {
+            if (!list.isEmpty() && list.get(0) instanceof Scalar) {
                 super.setActualInstance(instance);
                 return;
             }
diff --git a/samples/client/petstore/kotlin-model-prefix-type-mappings/src/main/kotlin/org/openapitools/client/models/ApiAnyOfUserOrPetOrArrayString.kt b/samples/client/petstore/kotlin-model-prefix-type-mappings/src/main/kotlin/org/openapitools/client/models/ApiAnyOfUserOrPetOrArrayString.kt
index 4f908f690796..6f349a94b4b8 100644
--- a/samples/client/petstore/kotlin-model-prefix-type-mappings/src/main/kotlin/org/openapitools/client/models/ApiAnyOfUserOrPetOrArrayString.kt
+++ b/samples/client/petstore/kotlin-model-prefix-type-mappings/src/main/kotlin/org/openapitools/client/models/ApiAnyOfUserOrPetOrArrayString.kt
@@ -74,7 +74,7 @@ data class ApiAnyOfUserOrPetOrArrayString(var actualInstance: Any? = null) {
                     // check if the actual instance is of the type `kotlin.collections.List<kotlin.String>`
                     if (value.actualInstance is List<*>) {
                         val list = value.actualInstance as List<Any>
-                        if (list.get(0) is kotlin.String) {
+                        if (!list.isEmpty() && list.get(0) is kotlin.String) {
                             val array = adapterkotlincollectionsListkotlinString.toJsonTree(value.actualInstance as kotlin.collections.List<kotlin.String>?).getAsJsonArray()
                             elementAdapter.write(out, array)
                             return
diff --git a/samples/client/petstore/kotlin-model-prefix-type-mappings/src/main/kotlin/org/openapitools/client/models/ApiUserOrPetOrArrayString.kt b/samples/client/petstore/kotlin-model-prefix-type-mappings/src/main/kotlin/org/openapitools/client/models/ApiUserOrPetOrArrayString.kt
index 060b98933e64..02d127bb854a 100644
--- a/samples/client/petstore/kotlin-model-prefix-type-mappings/src/main/kotlin/org/openapitools/client/models/ApiUserOrPetOrArrayString.kt
+++ b/samples/client/petstore/kotlin-model-prefix-type-mappings/src/main/kotlin/org/openapitools/client/models/ApiUserOrPetOrArrayString.kt
@@ -74,7 +74,7 @@ data class ApiUserOrPetOrArrayString(var actualInstance: Any? = null) {
                     // check if the actual instance is of the type `kotlin.collections.List<kotlin.String>`
                     if (value.actualInstance is List<*>) {
                         val list = value.actualInstance as List<Any>
-                        if (list.get(0) is kotlin.String) {
+                        if (!list.isEmpty() && list.get(0) is kotlin.String) {
                             val array = adapterkotlincollectionsListkotlinString.toJsonTree(value.actualInstance as kotlin.collections.List<kotlin.String>?).getAsJsonArray()
                             elementAdapter.write(out, array)
                             return