diff --git a/include/fmt/format.h b/include/fmt/format.h
index 9b4f7357..9909e19f 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -152,12 +152,21 @@ FMT_END_NAMESPACE
 
 #ifndef FMT_USE_USER_DEFINED_LITERALS
 // EDG based compilers (Intel, NVIDIA, Elbrus, etc), GCC and MSVC support UDLs.
+//
+// GCC before 4.9 requires a space in `operator"" _a` which is invalid in later
+// compiler versions.
 #  if (FMT_HAS_FEATURE(cxx_user_literals) || FMT_GCC_VERSION >= 407 || \
        FMT_MSC_VERSION >= 1900) &&                                     \
       (!defined(__EDG_VERSION__) || __EDG_VERSION__ >= /* UDL feature */ 480)
 #    define FMT_USE_USER_DEFINED_LITERALS 1
+#    if FMT_GCC_VERSION > 0 && FMT_GCC_VERSION < 409
+#      define FMT_WHITESPACE_IN_USER_DEFINED_LITERALS 1
+#    else
+#      define FMT_WHITESPACE_IN_USER_DEFINED_LITERALS 0
+#    endif
 #  else
 #    define FMT_USE_USER_DEFINED_LITERALS 0
+#    define FMT_WHITESPACE_IN_USER_DEFINED_LITERALS 0
 #  endif
 #endif
 
@@ -4427,10 +4436,14 @@ template <detail_exported::fixed_string Str> constexpr auto operator""_a() {
   using char_t = remove_cvref_t<decltype(Str.data[0])>;
   return detail::udl_arg<char_t, sizeof(Str.data) / sizeof(char_t), Str>();
 }
-#  else
+#  elif FMT_WHITESPACE_IN_USER_DEFINED_LITERALS
 constexpr auto operator"" _a(const char* s, size_t) -> detail::udl_arg<char> {
   return {s};
 }
+#  else
+constexpr auto operator""_a(const char* s, size_t) -> detail::udl_arg<char> {
+  return {s};
+}
 #  endif
 }  // namespace literals
 #endif  // FMT_USE_USER_DEFINED_LITERALS