diff --git a/include/argparse.hpp b/include/argparse.hpp
index faf1d74..560aaaa 100644
--- a/include/argparse.hpp
+++ b/include/argparse.hpp
@@ -1899,6 +1899,11 @@ namespace argparse
 
                     ~ArgumentBuilder() noexcept(false)
                     {
+                        if (is_mutually_exclusive() && !is_allowed_in_mutually_exclusive_group())
+                        {
+                            throw option_error("mutually exclusive arguments must be optional");
+                        }
+
                         if ((m_options.action == argparse::version) && m_options.help.empty())
                         {
                             m_options.help = "show program's version number and exit";
@@ -1906,25 +1911,10 @@ namespace argparse
 
                         if (is_positional())
                         {
-                            if (m_options.mutually_exclusive_group != nullptr
-                                && (!m_options.nargs.has_value()
-                                    || (!std::holds_alternative<Nargs>(*m_options.nargs)
-                                        || (std::get<Nargs>(*m_options.nargs) != zero_or_one
-                                            && std::get<Nargs>(*m_options.nargs) != zero_or_more))))
-                            {
-                                throw option_error("mutually exclusive arguments must be optional");
-                            }
-
                             m_arguments.emplace_back(PositionalArgument(std::move(m_options)));
                         }
                         else
                         {
-                            if (m_options.mutually_exclusive_group != nullptr
-                                && m_options.required)
-                            {
-                                throw option_error("mutually exclusive arguments must be optional");
-                            }
-
                             m_arguments.emplace_back(OptionalArgument(std::move(m_options)));
                         }
                     }
@@ -2025,6 +2015,40 @@ namespace argparse
                         return !m_options.names.front().starts_with('-');
                     }
 
+                    auto is_mutually_exclusive() const -> bool
+                    {
+                        return m_options.mutually_exclusive_group != nullptr;
+                    }
+
+                    auto is_allowed_in_mutually_exclusive_group() const -> bool
+                    {
+                        if (is_positional())
+                        {
+                            return is_optional_by_nargs_option();
+                        }
+                        else
+                        {
+                            return !m_options.required;
+                        }
+                    }
+
+                    auto is_optional_by_nargs_option() const -> bool
+                    {
+                        if (!m_options.nargs.has_value())
+                        {
+                            return false;
+                        }
+
+                        if (!std::holds_alternative<Nargs>(*m_options.nargs))
+                        {
+                            return false;
+                        }
+
+                        auto const nargs = std::get<Nargs>(*m_options.nargs);
+
+                        return nargs == zero_or_one || nargs == zero_or_more;
+                    }
+
                 private:
                     Arguments & m_arguments;
                     OptString & m_version;