diff --git a/src/disk.c b/src/disk.c
index 2db197b..a608693 100644
--- a/src/disk.c
+++ b/src/disk.c
@@ -221,7 +221,7 @@ int disk_dump_dev(int fd, const char * file) {
 #else
 
 	blksize = lseek(fd, 0, SEEK_END);
-	if ( blksize == (off_t)-1 ) {
+	if ( (off_t)blksize == (off_t)-1 ) {
 		ERROR_INFO("Cannot get size of block device");
 		return -1;
 	}