diff --git a/sys/net/gnrc/transport_layer/tcp/gnrc_tcp_pkt.c b/sys/net/gnrc/transport_layer/tcp/gnrc_tcp_pkt.c
index 67d53ea40f..ef23a43d03 100644
--- a/sys/net/gnrc/transport_layer/tcp/gnrc_tcp_pkt.c
+++ b/sys/net/gnrc/transport_layer/tcp/gnrc_tcp_pkt.c
@@ -455,7 +455,7 @@ int _pkt_acknowledge(gnrc_tcp_tcb_t *tcb, const uint32_t ack)
             /* If this is a subsequent sample */
             else {
                 tcb->rtt_var = (tcb->rtt_var / CONFIG_GNRC_TCP_RTO_B_DIV) * (CONFIG_GNRC_TCP_RTO_B_DIV-1);
-                tcb->rtt_var += abs(tcb->srtt - rtt) / CONFIG_GNRC_TCP_RTO_B_DIV;
+                tcb->rtt_var += labs(tcb->srtt - rtt) / CONFIG_GNRC_TCP_RTO_B_DIV;
                 tcb->srtt = (tcb->srtt / CONFIG_GNRC_TCP_RTO_A_DIV) * (CONFIG_GNRC_TCP_RTO_A_DIV-1);
                 tcb->srtt += rtt / CONFIG_GNRC_TCP_RTO_A_DIV;
             }