Trouble with setting a form error

Hello,

I am attempting to author a rule that is triggered when two fields are left blank on a node, posting a form error and preventing submission (the idea is that the author needs to select one or both, but can't leave both blank). The conditions for my rule are operating as expected (e.g. I can say 'dsm("works");' in the actions section and I get a response when expected). I have tried using all triggers under the node section of the triggers to no avail.

The rule is currently set up as:

Events

Before saving content

Conditions

Content is of type (Parameter: Content: [node], Content types: File)

User has role(s) (Parameter: User: [node:author], Roles: employee, project manager,..., Match roles: any)

Data value is empty (Parameter: Data to check: [node:group-audience])

AND

Data value is empty (Parameter: Data to check: [node:field-file-company])

Actions

Execute custom PHP code (Parameter: PHP code: form_set_error('field_file_company', t('It is required that you select either company level access, group level access, or both.'));

So, I get a form error, but it appears on the node being edited's display page *above* a message stating "*node name* has been updated.".

Any ideas why my form_set_error isn't preventing the form from submitting? I know I have the field names right. Thanks,

Rich