diff --git a/toonz/sources/toonz/xsheetviewer.cpp b/toonz/sources/toonz/xsheetviewer.cpp
index 6e45023..34d6a6b 100644
--- a/toonz/sources/toonz/xsheetviewer.cpp
+++ b/toonz/sources/toonz/xsheetviewer.cpp
@@ -1196,7 +1196,7 @@ QString XsheetViewer::getFrameNumberWithLetters(int frame) {
   } else
     number = "0";
 
-  return number.append(letter);
+  return (QChar(letter).isNull()) ? number : number.append(letter);
 }
 //-----------------------------------------------------------------------------