diff --git a/server/implementation/src/main/java/io/smallrye/graphql/execution/error/ExecutionErrorsService.java b/server/implementation/src/main/java/io/smallrye/graphql/execution/error/ExecutionErrorsService.java
index c95cf873e..917ec6715 100644
--- a/server/implementation/src/main/java/io/smallrye/graphql/execution/error/ExecutionErrorsService.java
+++ b/server/implementation/src/main/java/io/smallrye/graphql/execution/error/ExecutionErrorsService.java
@@ -110,7 +110,8 @@ private void populateCustomExtensions(JsonObjectBuilder objectBuilder, Map<Strin
                 if (config.getErrorExtensionFields().isEmpty()
                         || (config.getErrorExtensionFields().isPresent()
                                 && config.getErrorExtensionFields().get().contains(entry.getKey()))) {
-                    addKeyValue(objectBuilder, entry.getKey(), entry.getValue().toString());
+                    Object value = entry.getValue();
+                    addKeyValue(objectBuilder, entry.getKey(), value != null ? value.toString() : null);
                 }
             }
         }