diff --git a/src/pylorax/imgutils.py b/src/pylorax/imgutils.py
index d713e4c1..301bb523 100644
--- a/src/pylorax/imgutils.py
+++ b/src/pylorax/imgutils.py
@@ -141,7 +141,7 @@ def mkrootfsimg(rootdir, outfile, label, size=2, sysroot=""):
 def mksparse(outfile, size):
     '''use os.ftruncate to create a sparse file of the given size.'''
     with open(outfile, "w") as fobj:
-        os.ftruncate(fobj.fileno(), size)
+        os.ftruncate(fobj.fileno(), int(size))
 
 def mkqcow2(outfile, size, options=None):
     '''use qemu-img to create a file of the given size.