diff --git a/disk-utils/isosize.c b/disk-utils/isosize.c
index 0f676ca88..5ae5a9e1b 100644
--- a/disk-utils/isosize.c
+++ b/disk-utils/isosize.c
@@ -138,7 +138,7 @@ static void isosize(int argc, char *filenamep, int xflag, long divisor)
 	if (lseek(fd, 16 << 11, 0) == (off_t) - 1)
 		err(EXIT_FAILURE, _("seek error on %s"), filenamep);
 
-	if (read(fd, &ipd, sizeof(ipd)) < 0)
+	if (read(fd, &ipd, sizeof(ipd)) != (ssize_t) sizeof(ipd))
 		err(EXIT_FAILURE, _("read error on %s"), filenamep);
 
 	nsecs = isonum_733(ipd.volume_space_size, xflag);