diff --git a/compiler2/asn1/asn1p.y b/compiler2/asn1/asn1p.y
index 584b8f8e62f33a2a484466bde1ab3f850d2cb61d..1721d642ea289cc59467cbd2895c00927c212f3b 100644
--- a/compiler2/asn1/asn1p.y
+++ b/compiler2/asn1/asn1p.y
@@ -827,6 +827,9 @@ ModuleDefinition:
     $$=new Asn::Module($1, $4, $5, $8.exp, $8.imp, $8.asss);
     $$->set_location(asn1_infile, @1.first_line);
     delete $2;
+    if ($5) {
+      $$->warning("EXTENSIBILITY IMPLIED is not supported.");
+    }
   }
 ;