diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java
index 6b98b0d5e24..81d7f4ab9e1 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java
@@ -514,6 +514,10 @@ public class OpenAPINormalizer {
             return;
         }
 
+        if (schema.getAllOf() == null) {
+            return;
+        }
+
         for (Object item : schema.getAllOf()) {
             if (!(item instanceof Schema)) {
                 throw new RuntimeException("Error! allOf schema is not of the type Schema: " + item);