diff --git a/src/libknot/rrset.c b/src/libknot/rrset.c
index e0af264076c404436ba025536d4d440c20b24de3..48a1c6c60a6b17bd8edb43694ca66ce89f5d6b4a 100644
--- a/src/libknot/rrset.c
+++ b/src/libknot/rrset.c
@@ -263,6 +263,8 @@ size_t knot_rrset_size(const knot_rrset_t *rrset)
 	}
 
 	uint16_t rr_count = rrset->rrs.rr_count;
+
+	assert(rrset->owner);
 	size_t total_size = knot_dname_size(rrset->owner) * rr_count;
 
 	for (size_t i = 0; i < rr_count; ++i) {