diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index c2463612c53eab692191eaf82788ae238c7d9e78..dc0639f4f294c448da5ecc6f3d415f2a57e39437 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -1933,7 +1933,7 @@ hb_buffer_diff (hb_buffer_t *buffer,
       result |= HB_BUFFER_DIFF_FLAG_CODEPOINT_MISMATCH;
     if (buf_info->cluster != ref_info->cluster)
       result |= HB_BUFFER_DIFF_FLAG_CLUSTER_MISMATCH;
-    if (((buf_info->mask & HB_GLYPH_FLAG_DEFINED) & (ref_info->mask & HB_GLYPH_FLAG_DEFINED)) != (ref_info->mask & HB_GLYPH_FLAG_DEFINED))
+    if ((buf_info->mask & ~ref_info->mask & HB_GLYPH_FLAG_DEFINED))
       result |= HB_BUFFER_DIFF_FLAG_GLYPH_FLAGS_MISMATCH;
     if (contains && ref_info->codepoint == dottedcircle_glyph)
       result |= HB_BUFFER_DIFF_FLAG_DOTTED_CIRCLE_PRESENT;