diff --git a/cloudinit/config/cc_mounts.py b/cloudinit/config/cc_mounts.py
index e1c43e3..55b6770 100644
--- a/cloudinit/config/cc_mounts.py
+++ b/cloudinit/config/cc_mounts.py
@@ -226,17 +226,17 @@ def suggested_swapsize(memsize=None, maxsize=None, fsys=None):
 def create_swapfile(fname, size):
     """Size is in MiB."""
 
-    errmsg = "Failed to create swapfile '%s' of size %dMB via %s: %s"
+    errmsg = "Failed to create swapfile '%s' of size %sMB via %s: %s"
 
     def create_swap(fname, size, method):
         LOG.debug("Creating swapfile in '%s' on fstype '%s' using '%s'",
                   fname, fstype, method)
 
         if method == "fallocate":
-            cmd = ['fallocate', '-l', '%dM' % size, fname]
+            cmd = ['fallocate', '-l', '%sM' % size, fname]
         elif method == "dd":
             cmd = ['dd', 'if=/dev/zero', 'of=%s' % fname, 'bs=1M',
-                   'count=%d' % size]
+                   'count=%s' % size]
 
         try:
             util.subp(cmd, capture=True)
diff --git a/cloudinit/config/tests/test_mounts.py b/cloudinit/config/tests/test_mounts.py
new file mode 100644
index 0000000..c7dad61
--- /dev/null
+++ b/cloudinit/config/tests/test_mounts.py
@@ -0,0 +1,22 @@
+# This file is part of cloud-init. See LICENSE file for license information.
+from unittest import mock
+
+from cloudinit.config.cc_mounts import create_swapfile
+
+
+M_PATH = 'cloudinit.config.cc_mounts.'
+
+
+class TestCreateSwapfile:
+
+    @mock.patch(M_PATH + 'util.subp')
+    def test_happy_path(self, m_subp, tmpdir):
+        swap_file = tmpdir.join("swap-file")
+        fname = str(swap_file)
+
+        # Some of the calls to util.subp should create the swap file; this
+        # roughly approximates that
+        m_subp.side_effect = lambda *args, **kwargs: swap_file.write('')
+
+        create_swapfile(fname, '')
+        assert mock.call(['mkswap', fname]) in m_subp.call_args_list