UTF-8 Problem ?

Hi Listers,
I want to insert some german specific characters (umlaut characters) into a table, but I am getting the following
Error message:
postgres=# EXECUTE stmt (1, 1 , 1 , 'Grün') ;
ERROR: invalid UTF-8 byte sequence detected near byte 0xfc

Or

postgres=# EXECUTE stmt (1, 1 , 1 , 'MAßßtab') ;
ERROR: invalid UTF-8 byte sequence detected near byte 0xdf

Here are my object/statement definitions :

A) PREPARE stmt( int, int, int, varchar) as insert INTO part values ($1,$2,$3,$4);

B)
postgres=# \d+ part
Table "public.part"
Column | Type | Modifiers | Description
--------+------------------------+-----------+-------------
id1 | integer | not null |
id2 | integer | not null |
id3 | integer | not null |
filler | character varying(200) | |

C)

postgres=# l\l
List of databases
Name | Owner | Encoding
------------+-------+-----------
db1 | user1 | SQL_ASCII
postgres | pg | UTF8
template0 | pg | UTF8
template1 | pg | UTF8

How to solve my problem ?

Best Regards. Milen