diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractTypeScriptClientCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractTypeScriptClientCodegen.java
index 9f09d9c5cb14..c5206ec9cdb1 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractTypeScriptClientCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractTypeScriptClientCodegen.java
@@ -772,7 +772,11 @@ private String enumValuesToEnumTypeUnion(List<String> values, String dataType) {
     private String numericEnumValuesToEnumTypeUnion(List<Number> values) {
         List<String> stringValues = new ArrayList<>();
         for (Number value : values) {
-            stringValues.add(value.toString());
+            if (value == null) {
+                LOGGER.warn("An enum value was null. See https://github.com/swagger-api/swagger-core/issues/4223");
+            } else {
+                stringValues.add(value.toString());
+            }
         }
         return enumValuesToEnumTypeUnion(stringValues, "number");
     }
diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/N4jsClientCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/N4jsClientCodegen.java
index 2448f1dc7e00..7c706a1b5e97 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/N4jsClientCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/N4jsClientCodegen.java
@@ -595,7 +595,11 @@ private String enumValuesToEnumTypeUnion(List<String> values, String dataType) {
     private String numericEnumValuesToEnumTypeUnion(List<Number> values) {
         List<String> stringValues = new ArrayList<>();
         for (Number value : values) {
-            stringValues.add(value.toString());
+            if (value == null) {
+                LOGGER.warn("An enum value was null. See https://github.com/swagger-api/swagger-core/issues/4223");
+            } else {
+                stringValues.add(value.toString());
+            }
         }
         return enumValuesToEnumTypeUnion(stringValues, "number");
     }