1 files changed, 4 insertions, 1 deletions
diff --git a/src/parsing.h b/src/parsing.h
index 12029d290..15f22040d 100644
--- a/src/parsing.h
+++ b/src/parsing.h
@@ -251,7 +251,10 @@ struct UniqueNameMapper {
 
   Name sourceToUnique(Name sName) {
     if (labelMappings.find(sName) == labelMappings.end()) {
-      throw ParseException("label mismatch in sourceToUnique");
+      throw ParseException("bad label in sourceToUnique");
+    }
+    if (labelMappings[sName].empty()) {
+      throw ParseException("use of popped label in sourceToUnique");
     }
     return labelMappings[sName].back();
   }