diff --git a/lib/appliance-kcmdline.c b/lib/appliance-kcmdline.c
index 8b78655eb..092d37329 100644
--- a/lib/appliance-kcmdline.c
+++ b/lib/appliance-kcmdline.c
@@ -65,7 +65,7 @@
 static char *
 get_root_uuid_with_file (guestfs_h *g, const char *appliance)
 {
-  unsigned char magic[2], uuid[16];
+  unsigned char magic[4], uuid[16];
   char *ret;
   int fd;
 
@@ -74,6 +74,10 @@ get_root_uuid_with_file (guestfs_h *g, const char *appliance)
     perrorf (g, _("open: %s"), appliance);
     return NULL;
   }
+  if (read (fd, magic, 4) != 4 || !strncmp ((char *) magic, "QFI\xfb", 4)) {
+    /* No point looking for extfs signature in QCOW2 directly. */
+    return NULL;
+  }
   if (lseek (fd, 0x438, SEEK_SET) != 0x438) {
   magic_error:
     error (g, _("%s: cannot read extfs magic in superblock"), appliance);