optocoupler trouble

I'm getting strange results from an optocoupler (aka optoisolator, and
this is the kind with transistor output). In brief: nothing I do
seems to make the transistor "conduct", except very feebly.

Here's my setup:

Diode anode: 0V/1.07V, 0.4mA
Diode cathode: ground

Transistor collector: connected to a 330 Ohm resistor which is
connected to 4.92V.
Transistor emitter: ground
Transistor base: open

When the diode anode is at 0V, the transistor collector is, as one
would expect, at 4.92V.
But when I give the diode anode 1.07V, the transistor collector drops
to just 4.88V. I want it to go down to 0V and I don't understand why
that's not happening.
.

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