diff --git a/tests/qemu-iotests/015 b/tests/qemu-iotests/015
index bdafd2629341c67d591566f4266934b9fe0a1056..44c134f94873436332a16f5cf00b36e4133a3b35 100755
--- a/tests/qemu-iotests/015
+++ b/tests/qemu-iotests/015
@@ -56,7 +56,8 @@ echo "creating image"
 # for the refcount table. On the other hand this is big enough to cause a
 # refcount table growth when rewriting the image after creating one snapshot.
 size=36M
-_make_test_img -o cluster_size=1k $size
+CLUSTER_SIZE=1k
+_make_test_img $size
 
 # Create two snapshots which fill the image with two different patterns
 echo "creating first snapshot"
diff --git a/tests/qemu-iotests/common.rc b/tests/qemu-iotests/common.rc
index cdefafc62fa5ed26b5e866a2192661731c17cdc6..26811ca660b65a8c8d4a9a9106c2ea37e7313465 100644
--- a/tests/qemu-iotests/common.rc
+++ b/tests/qemu-iotests/common.rc
@@ -58,9 +58,11 @@ _make_test_img()
     # extra qemu-img options can be added by tests
     # at least one argument (the image size) needs to be added
     local extra_img_options=$*
+    local cluster_size_filter="s# cluster_size=[0-9]\\+##g"
 
     if [ \( "$IMGFMT" = "qcow2" -o "$IMGFMT" = "qed" \) -a -n "$CLUSTER_SIZE" ]; then
         extra_img_options="-o cluster_size=$CLUSTER_SIZE $extra_img_options"
+        cluster_size_filter=""
     fi
 
     # XXX(hch): have global image options?
@@ -69,7 +71,7 @@ _make_test_img()
     	sed -e "s#$TEST_DIR#TEST_DIR#g" | \
     	sed -e "s#$IMGFMT#IMGFMT#g" | \
 	sed -e "s# encryption=off##g" | \
-	sed -e "s# cluster_size=0##g" | \
+	sed -e "$cluster_size_filter" | \
 	sed -e "s# table_size=0##g" | \
 	sed -e "s# compat6=off##g" | \
 	sed -e "s# static=off##g"