Assignment behaves like Validation

Hi,

I have a very simple assignment where i want to assign a fixed text value to a text field. My assignment goes successful if i perform it with the text value that already exists in the field. Any other text fails the assignment. It looks like it is validating the field text with the new value. Moreover, the text on the expression editor also says "Validation Expression".

I am using MDM 5,5 SP4 Patch01. Would appreciate any help.

Regards,

Yasir Rizwan Saqib