diff --git a/tests/test-utils.c b/tests/test-utils.c
index c9b859d..41874a1 100644
--- a/tests/test-utils.c
+++ b/tests/test-utils.c
@@ -25,6 +25,8 @@
 
 /* A small implementation of TAP */
 static unsigned int test_number = 0;
+
+__attribute__((format(printf, 1, 2)))
 static void
 ok (const char *format, ...)
 {