ERROR 000539: Invalid field <field name>

Error Message

In some instances, running the Calculate Field tool to calculate the value of a field for a feature class or layer with a Python expression fails, and returns the following error message:

Error:   
ERROR 000539: Invalid field <field name>

Cause

This issue occurs if the field referenced in the Python expression does not exists or is misspelled. In this example, the ‘Number_of_Interns’ field is misspelled as 'Number_of_Intern' in the expression.

The correct field name is ‘Number_of_Interns’ as displayed in the attribute table below.

Clicking the Verify button before running the expression does not check if the field exists or is misspelled. Therefore, it indicates the expression is valid without any error message.

Solution or Workaround

Ensure or modify the Python expression in the Calculate Field pane to have the correct field name(s) before clicking Run. In this example, the expression references the correct field name of "Number_of_Interns", instead of the misspelled "Number_of_Intern".