diff --git a/tools/mkboot.c b/tools/mkboot.c
index e2d971d740..420ae248e6 100644
--- a/tools/mkboot.c
+++ b/tools/mkboot.c
@@ -26,7 +26,7 @@ void usage(void)
     exit(1);
 }
 
-char image[0x200000 + 0x220 + 0x200000/0x200];
+unsigned char image[0x200000 + 0x220 + 0x200000/0x200];
 
 int main(int argc, char *argv[])
 {