diff --git a/dns64/dns64.c b/dns64/dns64.c
index 4b3c6cee5..1e38b8f0d 100644
--- a/dns64/dns64.c
+++ b/dns64/dns64.c
@@ -225,6 +225,7 @@ ipv4_to_ptr(uint32_t ipv4, char ptr[], size_t nm_len)
     static const char IPV4_PTR_SUFFIX[] = "\07in-addr\04arpa";
     int i;
     char* c = ptr;
+    log_assert(nm_len == MAX_PTR_QNAME_IPV4);
 
     for (i = 0; i < 4; ++i) {
         *c = uitoa((unsigned int)(ipv4 % 256), c + 1);