diff --git a/src/lib/fmt/SocketAddressFormatter.cxx b/src/lib/fmt/SocketAddressFormatter.cxx
index bd31f969c..048ea981f 100644
--- a/src/lib/fmt/SocketAddressFormatter.cxx
+++ b/src/lib/fmt/SocketAddressFormatter.cxx
@@ -5,7 +5,7 @@
 #include "net/ToString.hxx"
 
 auto
-fmt::formatter<SocketAddress>::format(SocketAddress address, format_context &ctx)
+fmt::formatter<SocketAddress>::format(SocketAddress address, format_context &ctx) const
   -> format_context::iterator
 {
 	return formatter<string_view>::format(ToString(address), ctx);
diff --git a/src/lib/fmt/SocketAddressFormatter.hxx b/src/lib/fmt/SocketAddressFormatter.hxx
index 7020d9083..0d835190f 100644
--- a/src/lib/fmt/SocketAddressFormatter.hxx
+++ b/src/lib/fmt/SocketAddressFormatter.hxx
@@ -12,7 +12,7 @@
 template<>
 struct fmt::formatter<SocketAddress> : formatter<string_view>
 {
-	auto format(SocketAddress address, format_context &ctx) -> format_context::iterator;
+	auto format(SocketAddress address, format_context &ctx) const -> format_context::iterator;
 };
 
 template<std::convertible_to<SocketAddress> T>