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 daed144eb13d..f0797266d0f2 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
@@ -743,7 +743,17 @@ private boolean isNullTypeSchema(Schema schema) {
             return true;
         }
 
-        if ((schema.getType() == null || schema.getType().equals("null")) && schema.get$ref() == null) {
+        if (schema.getTypes() != null && !schema.getTypes().isEmpty()) {
+            // 3.1 spec
+            if (schema.getTypes().size() ==1) { // 1 type only
+                String type = (String) schema.getTypes().iterator().next();
+                return type == null || "null".equals(type);
+            } else { // more than 1 type so must not be just null
+                return false;
+            }
+        }
+
+        if ((schema.getType() == null || schema.getType().equals("null"))) {
             return true;
         }
 
diff --git a/modules/openapi-generator/src/test/resources/3_1/java/petstore.yaml b/modules/openapi-generator/src/test/resources/3_1/java/petstore.yaml
index 491785afdb81..c64385a963bc 100644
--- a/modules/openapi-generator/src/test/resources/3_1/java/petstore.yaml
+++ b/modules/openapi-generator/src/test/resources/3_1/java/petstore.yaml
@@ -889,3 +889,11 @@ components:
       properties: {}
       type: object
       title: HTTPValidationError
+    AnyOfArray:
+      anyOf:
+        - type: array
+          items:
+            type: string
+        - type: array
+          items:
+            type: integer
diff --git a/samples/client/petstore/java/okhttp-gson-3.1/.openapi-generator/FILES b/samples/client/petstore/java/okhttp-gson-3.1/.openapi-generator/FILES
index 0a065d67dc83..5ba46e594fb5 100644
--- a/samples/client/petstore/java/okhttp-gson-3.1/.openapi-generator/FILES
+++ b/samples/client/petstore/java/okhttp-gson-3.1/.openapi-generator/FILES
@@ -6,6 +6,7 @@ api/openapi.yaml
 build.gradle
 build.sbt
 docs/Animal.md
+docs/AnyOfArray.md
 docs/AnyTypeTest.md
 docs/Cat.md
 docs/Category.md
@@ -57,6 +58,7 @@ src/main/java/org/openapitools/client/auth/OAuthOkHttpClient.java
 src/main/java/org/openapitools/client/auth/RetryingOAuth.java
 src/main/java/org/openapitools/client/model/AbstractOpenApiSchema.java
 src/main/java/org/openapitools/client/model/Animal.java
+src/main/java/org/openapitools/client/model/AnyOfArray.java
 src/main/java/org/openapitools/client/model/AnyTypeTest.java
 src/main/java/org/openapitools/client/model/Cat.java
 src/main/java/org/openapitools/client/model/Category.java
diff --git a/samples/client/petstore/java/okhttp-gson-3.1/README.md b/samples/client/petstore/java/okhttp-gson-3.1/README.md
index 8e78bf3d184d..cb3486b20fb6 100644
--- a/samples/client/petstore/java/okhttp-gson-3.1/README.md
+++ b/samples/client/petstore/java/okhttp-gson-3.1/README.md
@@ -143,6 +143,7 @@ Class | Method | HTTP request | Description
 ## Documentation for Models
 
  - [Animal](docs/Animal.md)
+ - [AnyOfArray](docs/AnyOfArray.md)
  - [AnyTypeTest](docs/AnyTypeTest.md)
  - [Cat](docs/Cat.md)
  - [Category](docs/Category.md)
diff --git a/samples/client/petstore/java/okhttp-gson-3.1/api/openapi.yaml b/samples/client/petstore/java/okhttp-gson-3.1/api/openapi.yaml
index c6c895706207..5816eb63139e 100644
--- a/samples/client/petstore/java/okhttp-gson-3.1/api/openapi.yaml
+++ b/samples/client/petstore/java/okhttp-gson-3.1/api/openapi.yaml
@@ -956,6 +956,12 @@ components:
       properties: {}
       title: HTTPValidationError
       type: object
+    AnyOfArray:
+      anyOf:
+      - items:
+          type: string
+      - items:
+          type: integer
     updatePetWithForm_request:
       properties:
         name:
diff --git a/samples/client/petstore/java/okhttp-gson-3.1/src/main/java/org/openapitools/client/JSON.java b/samples/client/petstore/java/okhttp-gson-3.1/src/main/java/org/openapitools/client/JSON.java
index 407a96d3f1db..72223340f53d 100644
--- a/samples/client/petstore/java/okhttp-gson-3.1/src/main/java/org/openapitools/client/JSON.java
+++ b/samples/client/petstore/java/okhttp-gson-3.1/src/main/java/org/openapitools/client/JSON.java
@@ -129,6 +129,7 @@ private static Class getClassByDiscriminator(Map classByDiscriminatorValue, Stri
         gsonBuilder.registerTypeAdapter(OffsetDateTime.class, offsetDateTimeTypeAdapter);
         gsonBuilder.registerTypeAdapter(LocalDate.class, localDateTypeAdapter);
         gsonBuilder.registerTypeAdapter(byte[].class, byteArrayAdapter);
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.AnyOfArray.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.AnyTypeTest.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.Cat.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.Category.CustomTypeAdapterFactory());