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 378cee272b10..ffa903d7cd7b 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
@@ -908,10 +908,15 @@ private Schema processNormalize31Spec(Schema schema, Set<Schema> visitedSchemas)
             return schema;
         }
 
-        if (schema == null || schema.getTypes() == null) {
+        if (schema == null) {
             return null;
         }
 
+        if (schema.getTypes() == null || schema.getTypes().isEmpty()) {
+            schema.setNullable(true);
+            return schema;
+        }
+
         // process null
         if (schema.getTypes().contains("null")) {
             schema.setNullable(true);