diff --git a/togl.c b/togl.c
index b7f1bef..0e1190f 100644
--- a/togl.c
+++ b/togl.c
@@ -132,7 +132,7 @@
 /* pointer to Tk_SetClassProcs function in the stub table */
 
 static void (*SetClassProcsPtr)
-        _ANSI_ARGS_((Tk_Window, Tk_ClassProcs *, ClientData));
+        _ANSI_ARGS_((Tk_Window, const Tk_ClassProcs *, ClientData));
 #endif
 
 /*