Photodiode Dark Current Measurement

I am trying to measure the dark current of a photodiode

I have Chathode of the PD connected to 1Meg ohm resistor which is
connected to +2V
The annode is connected to 0V

I obtain the following measurements
Voltage accross Resistor: 1.39V
Voltage accross Photodiode : 0.82V

I have assumed that the Dark Current (or indeed any current value) is
(2 - (1.39+0.82)) / 1Meg

=> I obtain a dark current of 210nA. This figrue however is less by
more than an order of magnitude than that I am expecting.

Is my understaning wrong?

Please don't ask why I have this set up am not using Op-Amp circuity!

Many Thanks

Graeme

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