diff --git a/testing/scripts/build-guestimages b/testing/scripts/build-guestimages
index 8e3a9c62e..4ef881836 100755
--- a/testing/scripts/build-guestimages
+++ b/testing/scripts/build-guestimages
@@ -41,7 +41,7 @@ do_on_exit umount $LOOPDIR
 for host in $STRONGSWANHOSTS
 do
 	log_action "Creating guest image for $host"
-	execute "qemu-img create -b $ROOTIMG -f $IMGEXT $IMGDIR/$host.$IMGEXT" 0
+	execute "qemu-img create -b $ROOTIMG -f $IMGEXT -F $IMGEXT $IMGDIR/$host.$IMGEXT" 0
 	execute "qemu-nbd -c $NBDEV $IMGDIR/$host.$IMGEXT" 0
 	partprobe $NBDEV
 	execute "mount $NBDPARTITION $LOOPDIR" 0
diff --git a/testing/scripts/build-strongswan b/testing/scripts/build-strongswan
index 779ec8bb3..af1a53ee8 100755
--- a/testing/scripts/build-strongswan
+++ b/testing/scripts/build-strongswan
@@ -96,7 +96,7 @@ case "$GUEST" in
 "")
 	if [ ! -f "$ROOTIMG" -o "$REPLACE" ]; then
 		log_action "Creating root image $ROOTIMG"
-		execute "qemu-img create -b $BASEIMG -f $IMGEXT $ROOTIMG"
+		execute "qemu-img create -b $BASEIMG -f $IMGEXT -F $IMGEXT $ROOTIMG"
 		ALL_RECIPES=1
 	fi
 	log_action "Connecting root image to NBD device $NBDEV"