diff --git a/src/harfbuzz-gsub.c b/src/harfbuzz-gsub.c
index 30d914ac05f4ac588e8c8b4a8ba70ef909b1ac2b..067122d65753e6efdb48a8bc50012789caf908fb 100644
--- a/src/harfbuzz-gsub.c
+++ b/src/harfbuzz-gsub.c
@@ -4218,7 +4218,7 @@ static HB_Error  GSUB_Do_String_Lookup( HB_GSUBHeader* gsub,
 	if ( error == HB_Err_Not_Covered )
 	  buffer->in_pos--;
       }
-      while (buffer->in_pos);
+      while ((HB_Int) buffer->in_pos >= 0);
 
       return retError;