diff --git a/include/graaflib/algorithm/shortest_path/a_star.tpp b/include/graaflib/algorithm/shortest_path/a_star.tpp
index 3f8fc8e3..c39e6524 100644
--- a/include/graaflib/algorithm/shortest_path/a_star.tpp
+++ b/include/graaflib/algorithm/shortest_path/a_star.tpp
@@ -60,9 +60,10 @@ std::optional<graph_path<WEIGHT_T>> a_star_search(
 
       // A* search does not work on negative edge weights.
       if (edge_weight < 0) {
-        throw std::invalid_argument{fmt::format(
-            "Negative edge weight [{}] between vertices [{}] -> [{}].",
-            edge_weight, current.id, neighbor)};
+        throw std::invalid_argument{
+            "Negative edge weight [" + std::to_string(edge_weight) +
+            "] between vertices [" + std::to_string(current.id) + "] -> [" +
+            std::to_string(neighbor) + "]."};
       }
 
       // tentative_g_score is the distance from start to the neighbor through
diff --git a/test/graaflib/algorithm/shortest_path/a_star_test.cpp b/test/graaflib/algorithm/shortest_path/a_star_test.cpp
index e3c67e0a..ec27bd0e 100644
--- a/test/graaflib/algorithm/shortest_path/a_star_test.cpp
+++ b/test/graaflib/algorithm/shortest_path/a_star_test.cpp
@@ -193,7 +193,7 @@ TYPED_TEST(AStarShortestPathSignedTypesTest, AStarNegativeWeight) {
               ex.what(),
               fmt::format(
                   "Negative edge weight [{}] between vertices [{}] -> [{}].",
-                  -1, vertex_id_1, vertex_id_2));
+                  std::to_string(weight_t{-1}), vertex_id_1, vertex_id_2));
           throw;
         }
       },