diff --git a/src/net/addr/ip.cpp b/src/net/addr/ip.cpp
index 81d2a4a..fca752a 100644
--- a/src/net/addr/ip.cpp
+++ b/src/net/addr/ip.cpp
@@ -3,4 +3,5 @@
 namespace stdplus
 {
 template In4Addr FromStr<In4Addr>::operator()(std::string_view) const;
-}
+template char* ToStr<In4Addr>::operator()(char*, In4Addr) const noexcept;
+} // namespace stdplus