That should work, the only things that would prevent from working are
1) The field is returning false, meaning that an image has not been added for the field.
2) Another plugin is interfering with ACF
I looked at the free version of the plugin you mentioned and there doesn’t seem to be anything in it that should cause a problem, but there might be something in the paid premium version that might.
If you deactivate this plugin other plugin does the error go away?
