diff --git a/tests/unit-tests/test_ccm.c b/tests/unit-tests/test_ccm.c
index be3e57f5..4e1d28d4 100644
--- a/tests/unit-tests/test_ccm.c
+++ b/tests/unit-tests/test_ccm.c
@@ -97,7 +97,7 @@ t_test_dtls_encrypt_params(void) {
 static void
 t_test_dtls_decrypt_params(void) {
   size_t n;
-  int len;
+  ssize_t len;
 
   for (n = 0; n < sizeof(data)/sizeof(struct test_vector); ++n) {
     dtls_ccm_params_t params =