diff --git a/include/openmc/output.h b/include/openmc/output.h
index 1ece3de960c..549d2ae32ba 100644
--- a/include/openmc/output.h
+++ b/include/openmc/output.h
@@ -76,7 +76,7 @@ struct formatter<std::array<T, 2>> {
   }
 
   template<typename FormatContext>
-  auto format(const std::array<T, 2>& arr, FormatContext& ctx)
+  auto format(const std::array<T, 2>& arr, FormatContext& ctx) const
   {
     return format_to(ctx.out(), "({}, {})", arr[0], arr[1]);
   }
diff --git a/include/openmc/position.h b/include/openmc/position.h
index 11ea3764792..e6939dc0e46 100644
--- a/include/openmc/position.h
+++ b/include/openmc/position.h
@@ -221,7 +221,7 @@ namespace fmt {
 template<>
 struct formatter<openmc::Position> : formatter<std::string> {
   template<typename FormatContext>
-  auto format(const openmc::Position& pos, FormatContext& ctx)
+  auto format(const openmc::Position& pos, FormatContext& ctx) const
   {
     return formatter<std::string>::format(
       fmt::format("({}, {}, {})", pos.x, pos.y, pos.z), ctx);
diff --git a/vendor/fmt b/vendor/fmt
index 65ac626c585..0c9fce2ffef 160000
--- a/vendor/fmt
+++ b/vendor/fmt
@@ -1 +1 @@
-Subproject commit 65ac626c5856f5aad1f1542e79407a6714357043
+Subproject commit 0c9fce2ffefecfdce794e1859584e25877b7b592