diff --git a/server/drivers/sed1330.c b/server/drivers/sed1330.c
index 0cdf5f5..785c2d6 100644
--- a/server/drivers/sed1330.c
+++ b/server/drivers/sed1330.c
@@ -787,7 +787,7 @@ inline void
 sed1330_set_pixel(PrivateData * p, int x, int y, int value)
 {
 	unsigned int bytepos;
-	char bitmask;
+	unsigned char bitmask;
 
 	bytepos = y * p->bytesperline + x / p->cellwidth;
 	bitmask = 0x80 >> (x % p->cellwidth);