diff --git a/wmcalc/mask.xbm b/wmcalc/mask.xbm
index 85290d7..828bbf2 100644
--- a/wmcalc/mask.xbm
+++ b/wmcalc/mask.xbm
@@ -1,7 +1,7 @@
 #define mask_width 64
 #define mask_height 64
 
-static char mask_bits[] = {
+static unsigned char mask_bits[] = {
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0xfe, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x3f,
  0xfe, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x7f,
diff --git a/wmcalc/wmcalc.c b/wmcalc/wmcalc.c
index 2e6a8ba..38d0dbe 100644
--- a/wmcalc/wmcalc.c
+++ b/wmcalc/wmcalc.c
@@ -204,7 +204,7 @@ int main( int argc, char **argv ) {
 
   if ( ( pixmask = XCreateBitmapFromData(display,
 					 win,
-					 mask_bits,
+					 (char *)mask_bits,
 					 mask_width,
 					 mask_height) )  == 0 ) {
     error_handler(ERR_X_CREATE_BITMAP, NULL);