diff --git a/include/snipplib/containers/snplib_hmap.h b/include/snipplib/containers/snplib_hmap.h
index cac2b87..e4d0097 100644
--- a/include/snipplib/containers/snplib_hmap.h
+++ b/include/snipplib/containers/snplib_hmap.h
@@ -462,11 +462,13 @@ public:
 
     template <typename VT, typename UT>
         requires requires(VT v, UT u) { u = v; }
-    inline static auto trivial_conv_from = [](VT const& val) -> hmap_expected_t<UT> { return static_cast<UT>(val); };
+    inline static std::function<hmap_expected_t<UT>(VT const&)> trivial_conv_from =
+        [](VT const& val) -> hmap_expected_t<UT> { return static_cast<UT>(val); };
 
     template <typename VT, typename UT>
         requires requires(VT v, UT u) { v = u; }
-    inline static auto trivial_conv_to = [](UT const& uval) -> hmap_expected_t<VT> { return static_cast<VT>(uval); };
+    inline static std::function<hmap_expected_t<VT>(UT const&)> trivial_conv_to =
+        [](UT const& uval) -> hmap_expected_t<VT> { return static_cast<VT>(uval); };
 
     hmap() = default;