diff --git a/include/freetype/internal/services/svotval.h b/include/freetype/internal/services/svotval.h
index 88da77887..fa95ca489 100644
--- a/include/freetype/internal/services/svotval.h
+++ b/include/freetype/internal/services/svotval.h
@@ -29,7 +29,7 @@ FT_BEGIN_HEADER
 
 
   typedef FT_Error
-  (*otv_validate_func)( FT_Face    face,
+  (*otv_validate_func)( FT_Face volatile  face,
                         FT_UInt    ot_flags,
                         FT_Bytes  *base,
                         FT_Bytes  *gdef,