diff --git a/lib/isc/tests/timer_test.c b/lib/isc/tests/timer_test.c
index 92f5b82489..128ce76b93 100644
--- a/lib/isc/tests/timer_test.c
+++ b/lib/isc/tests/timer_test.c
@@ -13,6 +13,7 @@
 
 #if HAVE_CMOCKA
 
+#include <inttypes.h>
 #include <stdarg.h>
 #include <stddef.h>
 #include <setjmp.h>