diff --git a/net/6lowpan/iphc.c b/net/6lowpan/iphc.c
index e116d308a8df..37eaff3f7b69 100644
--- a/net/6lowpan/iphc.c
+++ b/net/6lowpan/iphc.c
@@ -1086,12 +1086,12 @@ static u8 lowpan_iphc_mcast_ctx_addr_compress(u8 **hc_ptr,
 					      const struct lowpan_iphc_ctx *ctx,
 					      const struct in6_addr *ipaddr)
 {
-	u8 data[6];
+	u8 data[6] = {};
 
 	/* flags/scope, reserved (RIID) */
 	memcpy(data, &ipaddr->s6_addr[1], 2);
 	/* group ID */
-	memcpy(&data[1], &ipaddr->s6_addr[11], 4);
+	memcpy(&data[2], &ipaddr->s6_addr[12], 4);
 	lowpan_push_hc_data(hc_ptr, data, 6);
 
 	return LOWPAN_IPHC_DAM_00;