A simple way around this issue would be to add an input variable with the the scene's current frame, then referencing it in the expression.
Your final setup will look like this:
The steps to set this up are as follows:
- Add an input variable
- Change the variable type to 'Single Property' (from the dropdown next to the variable name)
- Change the 'ID' type to 'Scene' (from the dropdown in front of the 'Prop:' label)
- Select your active scene from in the Scene ID property search menu (from default startup you'll only see one scene named 'Scene')
- Type 'frame_current' into the 'Path' property
- type 'var' (or whatever you've named your variable) into the expression field
Input Variables are the proper way to read from RNA properties when using Driver expressions. This way Blender knows to update the dependencies and ensure the driver functions properly.