diff --git a/src/core/include/mp-units/format.h b/src/core/include/mp-units/format.h
index cc158e74..d5fea4b6 100644
--- a/src/core/include/mp-units/format.h
+++ b/src/core/include/mp-units/format.h
@@ -47,8 +47,8 @@ namespace mp_units::detail {
 template<typename Char>
 [[nodiscard]] constexpr const Char* at_most_one_of(const Char* begin, const Char* end, std::string_view modifiers)
 {
-  const Char* const it = find_first_of(begin, end, modifiers.begin(), modifiers.end());
-  if (it != end && find_first_of(it + 1, end, modifiers.begin(), modifiers.end()) != end)
+  const Char* const it = mp_units::detail::find_first_of(begin, end, modifiers.begin(), modifiers.end());
+  if (it != end && mp_units::detail::find_first_of(it + 1, end, modifiers.begin(), modifiers.end()) != end)
     throw MP_UNITS_STD_FMT::format_error("only one of '" + std::string(modifiers) +
                                          "' unit modifiers may be used in the format spec");
   return it;