diff --git a/src/eap_server/eap_server_sim.c b/src/eap_server/eap_server_sim.c
index 66a087212..f8aa508ec 100644
--- a/src/eap_server/eap_server_sim.c
+++ b/src/eap_server/eap_server_sim.c
@@ -286,7 +286,7 @@ static struct wpabuf * eap_sim_build_reauth(struct eap_sm *sm,
 	 * Session-Id calculation after receiving response from the peer and
 	 * after all other checks pass. */
 	os_memcpy(data->reauth_mac,
-		  wpabuf_head(buf) + wpabuf_len(buf) - EAP_SIM_MAC_LEN,
+		  wpabuf_head_u8(buf) + wpabuf_len(buf) - EAP_SIM_MAC_LEN,
 		  EAP_SIM_MAC_LEN);
 
 	return buf;