diff --git a/test/src/enc_dec_test.cc b/test/src/enc_dec_test.cc
index da504df..79ed2ac 100644
--- a/test/src/enc_dec_test.cc
+++ b/test/src/enc_dec_test.cc
@@ -43,7 +43,7 @@ struct fmt::formatter<MovableType> : formatter<int>
 {
   // parse is inherited from formatter<string_view>.
   template<typename FormatContext>
-  auto format(const MovableType& val, FormatContext& ctx) {
+  auto format(const MovableType& val, FormatContext& ctx) const {
     return formatter<int>::format(val.val[0].v, ctx);
   }
 };