diff --git a/tool/src/org/antlr/v4/tool/interp/ParserInterpreter.java b/tool/src/org/antlr/v4/tool/interp/ParserInterpreter.java
index 7221a0f83..a118f4a99 100644
--- a/tool/src/org/antlr/v4/tool/interp/ParserInterpreter.java
+++ b/tool/src/org/antlr/v4/tool/interp/ParserInterpreter.java
@@ -64,7 +64,7 @@ public class ParserInterpreter {
 
 		@Override
 		public String getGrammarFileName() {
-			return null;
+			throw new UnsupportedOperationException("not implemented");
 		}
 
 		@Override