[General] Unable to identify an operator '=' for types 'numeric' and 'double precision' You will have to retype this query using and explicit cast

Server hardware:
Pentium 166mmx
96mb ram
4gb HD

Server Software:
Red Hat Linux 7.1
PostgreSQL 7.2

Development:
PowerBuilder 5.0

The story start when I try these statements (execute in psql):

CREATE TABLE testdecimal ( decimal_value DECIMAL(6,2));
INSERT INTO testdecimal VALUES(1.0);
INSERT INTO testdecimal VALUES(1.1);
SELECT * FROM testdecimal WHERE decimal_value = 1.0;

Error : Unable to identify an operator '=' for types 'numeric' and 'double
precision'
        You will have to retype this query using and explicit cast