diff --git a/src/truetype/ttgxvar.c b/src/truetype/ttgxvar.c
index b462263de115c79a1d3069f0cf013832f3808396..082bcc075448edd64f5fc01616d320694b1af6a6 100644
--- a/src/truetype/ttgxvar.c
+++ b/src/truetype/ttgxvar.c
@@ -42,6 +42,7 @@
 #include <ft2build.h>
 #include <freetype/internal/ftdebug.h>
 #include FT_CONFIG_CONFIG_H
+#include <freetype/internal/ftcalc.h>
 #include <freetype/internal/ftstream.h>
 #include <freetype/internal/sfnt.h>
 #include <freetype/tttags.h>
@@ -1065,7 +1066,7 @@
                 delta == 1 ? "" : "s",
                 vertical ? "VVAR" : "HVAR" ));
 
-    *avalue += delta;
+    *avalue = ADD_INT( *avalue, delta );
 
   Exit:
     return error;