diff --git a/tool/src/org/antlr/v4/semantics/AttributeChecks.java b/tool/src/org/antlr/v4/semantics/AttributeChecks.java
index d300a3ccf..cc2deb751 100644
--- a/tool/src/org/antlr/v4/semantics/AttributeChecks.java
+++ b/tool/src/org/antlr/v4/semantics/AttributeChecks.java
@@ -150,8 +150,8 @@ public class AttributeChecks implements ActionSplitterListener {
 				return; // $ids for ids+=ID etc...
 			}
 			if ( isolatedRuleRef(x.getText())!=null ) {
-//				errMgr.grammarError(ErrorType.ISOLATED_RULE_REF,
-//									g.fileName, x, x.getText(), expr);
+				errMgr.grammarError(ErrorType.ISOLATED_RULE_REF,
+									g.fileName, x, x.getText(), expr);
 				return;
 			}
 			errMgr.grammarError(ErrorType.UNKNOWN_SIMPLE_ATTRIBUTE,