diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
index 1f4f91a10bd..2874cbfd29b 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
@@ -4191,6 +4191,9 @@ public class DefaultCodegen implements CodegenConfig {
     protected Parameter getParameterFromRef(String ref, OpenAPI openAPI) {
         String parameterName = ref.substring(ref.lastIndexOf('/') + 1);
         Map<String, Parameter> parameterMap = openAPI.getComponents().getParameters();
+        if (parameterMap == null ) { // can't find the ref
+            throw new RuntimeException(ref + " not defined in the spec.");
+        }
         return parameterMap.get(parameterName);
     }