diff --git a/src/proto/syntax.pro b/src/proto/syntax.pro
index 0872546..cd21418 100644
--- a/src/proto/syntax.pro
+++ b/src/proto/syntax.pro
@@ -32,6 +32,7 @@
 void hl_set_bg_color_name(char_u *name);
 void hl_set_fg_color_name(char_u *name);
 int get_cterm_attr_idx(int attr, int fg, int bg);
+int get_gui_attr_idx(int attr, guicolor_T fg, guicolor_T bg);
 void clear_hl_tables(void);
 int hl_combine_attr(int char_attr, int prim_attr);
 attrentry_T *syn_gui_attr2entry(int attr);