diff --git a/src/truetype/ttinterp.c b/src/truetype/ttinterp.c
index e07981e06..731095ed0 100644
--- a/src/truetype/ttinterp.c
+++ b/src/truetype/ttinterp.c
@@ -5003,9 +5003,9 @@
 
 #ifdef TT_SUPPORT_SUBPIXEL_HINTING_INFINALITY
     /* Disable Type 2 Vacuform Rounds - e.g. Arial Narrow */
-    if ( SUBPIXEL_HINTING_INFINALITY &&
-         exc->ignore_x_mode          &&
-         FT_ABS( D ) == 64           )
+    if ( SUBPIXEL_HINTING_INFINALITY         &&
+         exc->ignore_x_mode                  &&
+         ( D < 0 ? NEG_LONG( D ) : D ) == 64 )
       D += 1;
 #endif /* TT_SUPPORT_SUBPIXEL_HINTING_INFINALITY */