diff --git a/convert.c b/convert.c
index 95e6a5244f..9b67649032 100644
--- a/convert.c
+++ b/convert.c
@@ -1549,7 +1549,7 @@ struct stream_filter {
 	struct stream_filter_vtbl *vtbl;
 };
 
-static int null_filter_fn(struct stream_filter *filter,
+static int null_filter_fn(struct stream_filter *filter UNUSED,
 			  const char *input, size_t *isize_p,
 			  char *output, size_t *osize_p)
 {
@@ -1568,7 +1568,7 @@ static int null_filter_fn(struct stream_filter *filter,
 	return 0;
 }
 
-static void null_free_fn(struct stream_filter *filter)
+static void null_free_fn(struct stream_filter *filter UNUSED)
 {
 	; /* nothing -- null instances are shared */
 }