Index: uspace/app/mkexfat/mkexfat.c
===================================================================
--- uspace/app/mkexfat/mkexfat.c	(revision 875bc8b08a3c4016031ca1b371780211a1e93fdc)
+++ uspace/app/mkexfat/mkexfat.c	(revision 8d8ad19a6649e82fd08c7f9ec1f7ec28a2456cc7)
@@ -787,4 +787,11 @@
 	dev_path = *argv;
 
+	if (!dev_path) {
+		printf(NAME ": Error, you must specify a valid block"
+		    " device.\n");
+		usage();
+		return 1;
+	}
+
 	printf("Device = %s\n", dev_path);