--- a/mailnews/mime/src/mimeenc.cpp
+++ b/mailnews/mime/src/mimeenc.cpp
@@ -749,17 +749,17 @@ mime_decode_yenc_buffer (MimeDecoderData
         dest ++;
       }
 
       // The assertion below is helpful, too
       if (outSize)
         *outSize = dest - line;
 
       /* Now write out what we decoded for this line. */
-      NS_ASSERTION(dest >= line && dest < src, "nothing to write!");
+      NS_ASSERTION(dest >= line && dest <= src, "nothing to write!");
       if (dest > line)
       {
         status = data->write_buffer (line, dest - line, data->closure);
         if (status < 0) /* abort */
           return status;
       }
 
       /* Reset the line so that we don't think it's partial next time. */