diff --git a/include/fmt/format.h b/include/fmt/format.h
index 35b9c1c5..4c8e32b7 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -2178,8 +2178,8 @@ template <typename Char> struct arg_ref {
 
   arg_id_kind kind;
   union value {
-    FMT_CONSTEXPR value() : index(0u) {}
-    FMT_CONSTEXPR value(int id) : index(id) {}
+    FMT_CONSTEXPR value() : index{0u} {}
+    FMT_CONSTEXPR value(int id) : index{id} {}
     FMT_CONSTEXPR value(basic_string_view<Char> n) : name(n) {}
 
     int index;
diff --git a/test/format-test.cc b/test/format-test.cc
index 67eb9be8..f5a35569 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -20,8 +20,14 @@
 #  include <windows.h>
 #endif
 
+// Check if fmt/format.h compiles with the X11 index macro defined.
+#define index(x, y) no nice things
+
 #include "fmt/color.h"
 #include "fmt/format.h"
+
+#undef index
+
 #include "gmock.h"
 #include "gtest-extra.h"
 #include "mock-allocator.h"