Make sure to add a Rules condition
Entity has field (related to your additional
site:current-user-field in your Rules condition). And make sure to add that
Entity has field condition BEFORE the condition you already have.
That's also what is mentioned in the Rules UI when you're adding a "data comparison" condition: somewhere it says "The data selector helps you drill down into the data available to Rules. To make entity fields appear in the data selector, you may have to use the condition 'entity has field' (or 'content is of type')".
For a video tutorial that illustrates the importance of this
Entity has field condition, refer to Data types and data selection, especially what is shown between about 13:30 and 17:30 in it.