Can't set minBE and MacBE

Hello, i’m trying to set the minBE and maxBE with the following functions:

uint8_t csma_minBE = 2;

uint8_t csma_maxBE = 4;

gnrc_netapi_set(iface, NETOPT_CSMA_MINBE, 0, (uint8_t *)&csma_minBE, sizeof(uint8_t));

gnrc_netapi_set(iface, NETOPT_CSMA_MAXBE , 0, (uint8_t *)&csma_maxBE, sizeof(uint8_t));

Unfortunately, it doesn’t work. The values are not set. The function returns -134 value. I work with IoT-Lab m3 knots. What could be the reason? Thank You!