CREATE AGGREGATE state function with one argument

In the manual fro creating aggregate functions (http://www.postgresql.org/idocs/index.php?sql-createaggregate.html) it reads:

sfunc

The name of the state transition function to be called for each input data value. This is normally a function of two arguments, the first being of type state_type and the second of type input_data_type. --->Alternatively, for an aggregate that does not examine its input values, the function takes just one argument of type state_type<---. In either case the function must return a value of type state_type. This function takes the current state value and the current input data item, and returns the next state value.

So I try this:

thalis=# CREATE FUNCTION state_func(int4) RETURNS int4 AS 'SELECT $1;' LANGUAGE 'sql';
CREATE
thalis=# CREATE AGGREGATE myaggr (basetype=int4,sfunc=state_func,stype=int4);
ERROR: AggregateCreate: function 'state_func(int4, int4)' does not exist

I understand that I can have a state function that takes just one argument. Missing smthng?

TIA,
thalis

ps running 7.1.1