diff --git a/src/libvterm/t/harness.c b/src/libvterm/t/harness.c
index 5fa4cfb..75869d7 100644
--- a/src/libvterm/t/harness.c
+++ b/src/libvterm/t/harness.c
@@ -65,7 +65,7 @@
 
 static VTermEncodingInstance encoding;
 
-static void term_output(const char *s, size_t len, void *user)
+static void term_output(const char *s, size_t len, void *user UNUSED)
 {
   size_t i;