2018A33 step up converter

I am experimenting with the 2018 step up converter.
The circuit is available from aliexpress , most everywhere.

There is a inductance, a Skottky diode, and capacitance, and the
circuit. The 2018 cuts the coil short to earth. The diode fill the
capacitor and the output is measured for a feedback.
Textbook step up. Rock solid 5 Volt.

Now I have this (apparently) wild idea. I add another diode reversed
and a capacitor, to prey on the output. The circuit is not phased at all,
solid 5.08 V as far is it original function.
But I get a couple of negative millivolt out.

What am I doing wrong? Or should this work?
I checked this over and over again.

Groetjes Albert
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