diff --git a/lib/ntlm/ntlm.c b/lib/ntlm/ntlm.c
index eab7fae81..224869099 100644
--- a/lib/ntlm/ntlm.c
+++ b/lib/ntlm/ntlm.c
@@ -1105,7 +1105,7 @@ heim_ntlm_verify_ntlm2(const void *key, size_t len,
     HMAC_CTX_init(&c);
     HMAC_Init_ex(&c, ntlmv2, 16, EVP_md5(), NULL);
     HMAC_Update(&c, serverchallange, 8);
-    HMAC_Update(&c, ((char *)answer->data) + 16, answer->length - 16);
+    HMAC_Update(&c, ((unsigned char *)answer->data) + 16, answer->length - 16);
     HMAC_Final(&c, serveranswer, &hmaclen);
     HMAC_CTX_cleanup(&c);