diff --git a/modules/testcontainers/src/main/java/org/microshed/testing/testcontainers/ApplicationContainer.java b/modules/testcontainers/src/main/java/org/microshed/testing/testcontainers/ApplicationContainer.java
index d20b178c..34af9848 100644
--- a/modules/testcontainers/src/main/java/org/microshed/testing/testcontainers/ApplicationContainer.java
+++ b/modules/testcontainers/src/main/java/org/microshed/testing/testcontainers/ApplicationContainer.java
@@ -23,6 +23,8 @@
 import java.lang.annotation.Annotation;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
+import java.net.MalformedURLException;
+import java.net.URL;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
@@ -444,11 +446,25 @@ public void setWaitStrategy(WaitStrategy waitStrategy) {
      * Configures the application container with the supplied MicroProfile REST Client class that
      * will reference the supplied {@code hostUrl}
      *
-     * @param restClientClass The MicroProfile REST Client class
+     * @param restClientClass The MicroProfile REST Client interface, which must be annotated with
+     *            <code>@RegisterRestClient</code>
      * @param hostUrl The URL that the {@code restClientClass} will act as a REST client for
+     * @throws IllegalArgumentException If the provided restClientClass is not an interface or not
+     *             annotated with <code>@RegisterRestClient</code>
+     * @throws IllegalArgumentException If hostUrl is not a valid URL
      * @return the current instance
      */
     public ApplicationContainer withMpRestClient(Class<?> restClientClass, String hostUrl) {
+        Objects.requireNonNull(restClientClass, "restClientClass must be non-null");
+        Objects.requireNonNull(hostUrl, "hostUrl must be non-null");
+        if (!restClientClass.isInterface()) {
+            throw new IllegalArgumentException("Provided restClientClass " + restClientClass.getCanonicalName() + " must be an interface");
+        }
+        try {
+            new URL(hostUrl);
+        } catch (MalformedURLException e) {
+            throw new IllegalArgumentException(e);
+        }
         String configToken = readMpRestClientConfigKey(restClientClass);
         if (configToken == null || configToken.isEmpty())
             configToken = restClientClass.getCanonicalName();
@@ -467,9 +483,8 @@ private String readMpRestClientConfigKey(Class<?> restClientClass) {
                                                                              false,
                                                                              getClass().getClassLoader());
         } catch (ClassNotFoundException | LinkageError notFound) {
-            return null;
+            throw new ExtensionConfigurationException("Unable to load @RegisterRestClient", notFound);
         }
-
         Method getConfigKey = null;
         try {
             getConfigKey = RegisterRestClient.getMethod("configKey");
@@ -479,14 +494,16 @@ private String readMpRestClientConfigKey(Class<?> restClientClass) {
         }
 
         Optional<Annotation> foundAnno = (Optional<Annotation>) AnnotationSupport.findAnnotation(restClientClass, RegisterRestClient);
-        if (!foundAnno.isPresent())
-            return null;
+        if (!foundAnno.isPresent()) {
+            throw new IllegalArgumentException("Provided restClientClass " + restClientClass + " must be annotated with "
+                                               + RegisterRestClient.getSimpleName());
+        }
 
         Annotation anno = foundAnno.get();
         try {
             return (String) getConfigKey.invoke(anno);
         } catch (IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
-            return null;
+            throw new IllegalArgumentException("Unable to obtain configKey from " + anno + " found on " + restClientClass.getCanonicalName());
         }
     }
 
@@ -496,6 +513,7 @@ private String readMpRestClientConfigKey(Class<?> restClientClass) {
      *
      * @param restClientClass The MicroProfile REST Client class
      * @param hostUrl The URL that the {@code restClientClass} will act as a REST client for
+     * @throws IllegalArgumentException If hostUrl is not a valid URL
      * @return the current instance
      */
     public ApplicationContainer withMpRestClient(String restClientClass, String hostUrl) {
@@ -506,6 +524,11 @@ public ApplicationContainer withMpRestClient(String restClientClass, String host
         } else {
             restClientClass += "/mp-rest/url";
         }
+        try {
+            new URL(hostUrl);
+        } catch (MalformedURLException e) {
+            throw new IllegalArgumentException(e);
+        }
         return withEnv(restClientClass, hostUrl);
     }
 
diff --git a/modules/testcontainers/src/test/java/org/microshed/testing/testcontainers/ApplicationContainerTest.java b/modules/testcontainers/src/test/java/org/microshed/testing/testcontainers/ApplicationContainerTest.java
index edeab572..a96b76c6 100644
--- a/modules/testcontainers/src/test/java/org/microshed/testing/testcontainers/ApplicationContainerTest.java
+++ b/modules/testcontainers/src/test/java/org/microshed/testing/testcontainers/ApplicationContainerTest.java
@@ -19,6 +19,7 @@
 package org.microshed.testing.testcontainers;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.Arrays;
 import java.util.Map;
@@ -31,17 +32,24 @@ public class ApplicationContainerTest {
 
     // Base uri configured with: com_example_StringRestClient_mp_rest_url
     @RegisterRestClient
-    public static class SampleRestClient1 {
+    public static interface SampleRestClient1 {
     }
 
     // Base uri configured with: rc_config_key_mp_rest_url
     @RegisterRestClient(configKey = "rc-config-key")
-    public static class SampleRestClient2 {
+    public static interface SampleRestClient2 {
     }
 
     // Base uri configured with: CLIENT_CONFIG_KEY_mp_rest_url
     @RegisterRestClient(configKey = "CLIENT_CONFIG_KEY")
-    public static class SampleRestClient3 {
+    public static interface SampleRestClient3 {
+    }
+
+    public static interface NoAnnotationClient {
+    }
+
+    @RegisterRestClient
+    public static class ConcreteClassClient {
     }
 
     @Test
@@ -61,6 +69,19 @@ public void testMpRestClient() {
         assertEquals(clientUrl, appEnv.get("CLIENT_CONFIG_KEY/mp-rest/url"), appEnv.toString());
     }
 
+    @Test
+    public void testMpRestClientValidation() {
+        final String clientUrl = "http://example.com";
+
+        assertThrows(IllegalArgumentException.class, () -> dummyApp().withMpRestClient(NoAnnotationClient.class, clientUrl));
+        assertThrows(IllegalArgumentException.class, () -> dummyApp().withMpRestClient(ConcreteClassClient.class, clientUrl));
+        assertThrows(NullPointerException.class, () -> dummyApp().withMpRestClient(SampleRestClient1.class, null));
+        Class<?> nullClass = null;
+        assertThrows(NullPointerException.class, () -> dummyApp().withMpRestClient(nullClass, clientUrl));
+        assertThrows(IllegalArgumentException.class, () -> dummyApp().withMpRestClient(SampleRestClient1.class, "bogus"));
+        assertThrows(IllegalArgumentException.class, () -> dummyApp().withMpRestClient("com.example.StringRestClient/mp-rest/url", "bogus"));
+    }
+
     @Test
     public void testCorrectServerAdapter() {
         ApplicationContainer app = dummyApp();