diff --git a/x11vnc.c b/x11vnc.c
index 6559183..ee21d03 100644
--- a/x11vnc.c
+++ b/x11vnc.c
@@ -101,7 +101,7 @@ char ModifierState = 0;
 
 /* this function adjusts the modifiers according to mod (as from modifiers) and ModifierState */
 
-void tweakModifiers(char mod,Bool down)
+void tweakModifiers(signed char mod,Bool down)
 {
   Bool isShift=ModifierState&(LEFTSHIFT|RIGHTSHIFT);
 #ifdef KBDDEBUG