diff --git a/src/truetype/ttgload.c b/src/truetype/ttgload.c
index 39d9c3f..259bb2e 100644
--- a/src/truetype/ttgload.c
+++ b/src/truetype/ttgload.c
@@ -1848,6 +1848,13 @@

         limit = (short)gloader->current.num_subglyphs;

+        /* make sure this isn't negative as we're going to add 4 later */
+        if ( limit < 0 )
+        {
+          error = FT_THROW( Invalid_Argument );
+          goto Exit;
+        }
+
         /* construct an outline structure for              */
         /* communication with `TT_Vary_Apply_Glyph_Deltas' */
         outline.n_points   = (short)( gloader->current.num_subglyphs + 4 );