How to update value of LAST_AUDIT_DATE

Hi,

Due to some business requirements, we created a custom collection for audit.

Now I want to update the OOB field LAST_AUDIT_DATE with a value from the custom collection as this is used in an existing report. I am trying to set the value in my script using the syntax below:

doc.getFieldMetadata("LAST_AUDIT_DATE").set(doc, <someauditDate>);

But my script throws an Nullpointer exception stating  Attempt to invoke method set on null value.

What am I missing? Do I need to access this field by some other way?

Thanks,

Priyanka