diff --git a/example.c b/example.c
index 83781b4..63029ed 100644
--- a/example.c
+++ b/example.c
@@ -9,9 +9,9 @@ int HMAC_SHA256(const unsigned char *data, size_t data_len,
                 unsigned char *out, size_t *out_len)
 {
     unsigned int len = 0;
-    HMAC(EVP_sha256(), key, key_len, data, data_len, out, &len);
+    char *ac = HMAC(EVP_sha256(), key, key_len, data, data_len, out, &len);
     *out_len = len;
-    return 0;
+    return (ac != NULL) ? 0 : -1;
 }
 
 int main()
diff --git a/test.c b/test.c
index 7d5d2c8..727f861 100644
--- a/test.c
+++ b/test.c
@@ -1,4 +1,3 @@
-#include <stdlib.h>
 #include <openssl/hmac.h>
 #include <openssl/sha.h>
 #include <check.h>
@@ -10,9 +9,9 @@ int HMAC_SHA256(const unsigned char *data, size_t data_len,
                 unsigned char *out, size_t *out_len)
 {
   unsigned int len = 0;
-  HMAC(EVP_sha256(), key, key_len, data, data_len, out, &len);
+  char *ac = HMAC(EVP_sha256(), key, key_len, data, data_len, out, &len);
   *out_len = len;
-  return 0;
+  return (ac != NULL) ? 0 : -1;
 }
 
 START_TEST(AwsSigv4Test_AwsSigv4Sign)