diff --git a/src/libcharon/sa/ikev2/keymat_v2.c b/src/libcharon/sa/ikev2/keymat_v2.c
index b57306b66..fce0840e3 100644
--- a/src/libcharon/sa/ikev2/keymat_v2.c
+++ b/src/libcharon/sa/ikev2/keymat_v2.c
@@ -528,6 +528,7 @@ METHOD(keymat_v2_t, derive_child_keys, bool,
 			case ENCR_AES_GCM_ICV16:
 			case ENCR_AES_CTR:
 			case ENCR_NULL_AUTH_AES_GMAC:
+			case ENCR_CHACHA20_POLY1305:
 				enc_size += 4;
 				break;
 			default: