diff --git a/src/test/test_util.c b/src/test/test_util.c
index 6ecff6f1c3a2ccc5098d1650aba463db5503233b..b4d8a4d7670755b9c338e0944e8a40a7c13d2335 100644
--- a/src/test/test_util.c
+++ b/src/test/test_util.c
@@ -6190,7 +6190,7 @@ test_util_map_anon_nofork(void *arg)
   tor_munmap_anonymous(ptr, sz);
   ptr = tor_mmap_anonymous(sz, ANONMAP_NOINHERIT, &inherit);
   tt_ptr_op(ptr, OP_NE, 0);
-  memset(ptr, TEST_VALUE, sz);
+  memset(ptr, (uint8_t)TEST_VALUE, sz);
 
   tt_int_op(0, OP_EQ, pipe(pipefd));
   pid_t child = fork();