std::bad_function_call if no param arg in place of param arg

[igagis]

clargs::parser p;

p.add('a', "aaa", "parametrized arg", [](std::string_view v){});

p.parse(...);

run from cmd line as:

exec --aaa

Result:
std::bad_function_call is thrown.

Expected result:
std::invalid_argument is thrown saying that the arg requires a parameter.

Note:
running as

exec --aaa=1

gives no error