Problem using new Python type: "cannot add type to string"

[Janet]

Hi,

I created a new Python type (extension in C), and tried to call a
function with an object of the new type as the argument:

PyObject *arglist = PyTuple_New(1);
PyTuple_SetItem(arglist, 0, myObj); /* myObj is an object of the new
type */
PyObject_CallObject(funcObj, arglist);...

However,  the PyObject_CallObject() failed with the error message
saying that: "cannot add type <my new type name> to string".  It seems
that the error was generated when string_concat() in
Objects/stringobject.c is invoked.  The function tested on the type of
the second argument and outputted error whenever the argument is not a
PyStringObject.
I would like to know why does this problem appear.  Is there any
methods in the new type that I forgot to implement? (BTW, I did
implemented the tp_str, tp_repr, and tp_print in my new type).

Thanks