The 'hover' help for Alternate view indicates the field is optional, but when attempting to save the record it is required.
Page is "Page Business Rule"
Text of 'Hover':
Using an alternate view will allow for a different set of data when the workflow is created at runtime. The view will still be restricted by the current record's primary keys. If multiple records are returned, then multiple workflows will be sent out.
Field Name: AlternateView