Uppercase with Umlaud (¨)

Hello,

I'm having problem getting the correct result in my query; because the field I'm searching contains an Umlaud (¨)

This is what the query looks like.

SELECT FIELD1
FROM TABLE1
WHERE (UPPER(FIELD1) LIKE = 'ZÜLZ')

Field1 contains 'Zülz', so in lower case.

Firebird 2.5; Charset WIN1252; and ANSI_CHARSET.

What I find odd is that if I query the following in firebird, I get the correct result ('ZÜLZ')

SELECT UPPER('Zölz')
FROM RDB$DATABASE

but, if I do

SELECT UPPER(FIELD1)
FROM TABLE1
I get 'ZöLZ'; which is clearly wrong.

Can anyone shed a light on what's going wrong?

Thank you!