diff --git a/libctf/ctf-serialize.c b/libctf/ctf-serialize.c
index c6b8b49..ecbc617 100644
--- a/libctf/ctf-serialize.c
+++ b/libctf/ctf-serialize.c
@@ -1319,7 +1319,7 @@
 	{
 	  ctf_set_errno (fp, ENOMEM);
 	  ctf_err_warn (fp, 0, 0, _("ctf_write_mem: cannot allocate %li bytes"),
-			(unsigned long) fp->ctf_size + sizeof (struct ctf_header));
+			(unsigned long) (fp->ctf_size + sizeof (struct ctf_header)));
 	  return NULL;
 	}
       ctf_flip_header (hp);