Casting varchar to timestamp fails in plpgsql

While the following cast works in psql, an equivalent cast fails in plpgsql:

   select cast ('Dec 14 1901 08:45:52' as timestamp);

The following equivalent plgsql function

   create function rdbms_date(varchar) returns timestamp as '
   declare
     p_raw_date alias for $1;
   begin
     return cast (p_raw_date as timestamp);
   end;' language 'plpgsql';

fails with error message:

   ERROR: Cannot cast type 'varchar' to 'timestamp'

when called like so:

   select rdbms_date('Dec 14 1901 08:45:52');

Can someone explain how to cast a varchar to a timestamp in plpgsql?

Thanks,
Bart