Type modifier parameter of input function

Hi,

whenever pg calls my input function, the type modifier parameter is 
ALWAYS (-1). If I specify a type modifier like

SELECT 'Hello World!'::my_string_type(MODIFIER1,MODIFIER2);

then pg:
1. calls the my_string_type-typmod_in function, and gets the correct result
2. calls the my_string_type-input function with type modifier parameter (-1)
3. calls the CAST(my_string_type AS my_string_type) function with the 
correct type modifier returned by the typmod_in function and the 
my_string_type pointer returned by the input function.

How can I convince pg to call the input function with the correct type 
modifier?

Thanks in advance

Regards

Michael