The 'FEE' error message sometimes pops up even when the aperture of the host lens is set to the minimum. This can occur in particular for well-used lenses which may not completely engage and press down the tab on the FTZ due to wear. The solution is simple, use a small toothpick or similar and push down the sensor tab of the FTZ. Or even better, lock the tab permanently by inserting a tiny screw in the tab slot, then secure with a drop of glue or nail polish. All my FTZ adapters (4 at present) are modified in this manner.
Apart from avoiding the annoying error message, one now has the opportunity to see the depth of field and bokeh at apertures from f/5.6 to the maximum of the lens, for example, f/2 or f/1.2 before pushing the shutter release.