diff --git a/src/core/include/mp-units/ext/fixed_string.h b/src/core/include/mp-units/ext/fixed_string.h
index 0fde61ed..048cf2c2 100644
--- a/src/core/include/mp-units/ext/fixed_string.h
+++ b/src/core/include/mp-units/ext/fixed_string.h
@@ -265,13 +265,13 @@ public:
 };
 
 // deduction guides
-template<one_of<char, char8_t, char16_t, char32_t, wchar_t> CharT, std::convertible_to<CharT>... Rest>
+template<typename CharT, std::convertible_to<CharT>... Rest>
 basic_fixed_string(CharT, Rest...) -> basic_fixed_string<CharT, 1 + sizeof...(Rest)>;
 
 template<typename CharT, std::size_t N>
 basic_fixed_string(const CharT (&str)[N]) -> basic_fixed_string<CharT, N - 1>;
 
-template<one_of<char, char8_t, char16_t, char32_t, wchar_t> CharT, std::size_t N>
+template<typename CharT, std::size_t N>
 basic_fixed_string(std::from_range_t, std::array<CharT, N>) -> basic_fixed_string<CharT, N>;
 
 // specialized algorithms