diff --git a/server/src/main/java/org/elasticsearch/index/mapper/ObjectMapper.java b/server/src/main/java/org/elasticsearch/index/mapper/ObjectMapper.java
index 0bc19461719ec..922beb25d8a2f 100644
--- a/server/src/main/java/org/elasticsearch/index/mapper/ObjectMapper.java
+++ b/server/src/main/java/org/elasticsearch/index/mapper/ObjectMapper.java
@@ -533,15 +533,19 @@ private static Map<String, Mapper> buildMergedMappers(
             MergeReason reason,
             MapperMergeContext objectMergeContext
         ) {
-            Map<String, Mapper> mergedMappers = null;
-            for (Mapper mergeWithMapper : mergeWith) {
-                Mapper mergeIntoMapper = (mergedMappers == null ? existing.mappers : mergedMappers).get(mergeWithMapper.simpleName());
+            Iterator<Mapper> iterator = mergeWith.iterator();
+            if (iterator.hasNext() == false) {
+                return Map.copyOf(existing.mappers);
+            }
+            Map<String, Mapper> mergedMappers = new HashMap<>(existing.mappers);
+            while (iterator.hasNext()) {
+                Mapper mergeWithMapper = iterator.next();
+                Mapper mergeIntoMapper = mergedMappers.get(mergeWithMapper.simpleName());
 
-                Mapper merged;
                 if (mergeIntoMapper == null) {
-                    merged = mergeWithMapper;
+                    mergedMappers.put(mergeWithMapper.simpleName(), mergeWithMapper);
                 } else if (mergeIntoMapper instanceof ObjectMapper objectMapper) {
-                    merged = objectMapper.merge(mergeWithMapper, reason, objectMergeContext);
+                    mergedMappers.put(objectMapper.simpleName(), objectMapper.merge(mergeWithMapper, reason, objectMergeContext));
                 } else {
                     assert mergeIntoMapper instanceof FieldMapper || mergeIntoMapper instanceof FieldAliasMapper;
                     if (mergeWithMapper instanceof NestedObjectMapper) {
@@ -553,22 +557,13 @@ private static Map<String, Mapper> buildMergedMappers(
                     // If we're merging template mappings when creating an index, then a field definition always
                     // replaces an existing one.
                     if (reason == MergeReason.INDEX_TEMPLATE) {
-                        merged = mergeWithMapper;
+                        mergedMappers.put(mergeWithMapper.simpleName(), mergeWithMapper);
                     } else {
-                        merged = mergeIntoMapper.merge(mergeWithMapper, objectMergeContext);
+                        mergedMappers.put(mergeWithMapper.simpleName(), mergeIntoMapper.merge(mergeWithMapper, objectMergeContext));
                     }
                 }
-                if (mergedMappers == null) {
-                    mergedMappers = new HashMap<>(existing.mappers);
-                }
-                mergedMappers.put(merged.simpleName(), merged);
-            }
-            if (mergedMappers != null) {
-                mergedMappers = Map.copyOf(mergedMappers);
-            } else {
-                mergedMappers = Map.copyOf(existing.mappers);
             }
-            return mergedMappers;
+            return Map.copyOf(mergedMappers);
         }
     }