goforget: "Failed to update" error before complete decay

I just started playing with this awesome tool, however I'm facing an issue: my bins get zero-ed pre-maturely after a while. This is all it takes to reproduce the issue for me:

http://127.0.0.1:8090/incr?distribution=topics&field=Foo&N=200
http://127.0.0.1:8090/incr?distribution=topics&field=Bar&N=100

Then refreshing "http://127.0.0.1:8090/dist?distribution=topics" periodically, I can see both bins decaying at a logical rate. However, after a while (with both bins still > 0 in score and probability) both bins become count:0 and p:0 and "Failed to update: topics" appears in the log.

Any clue what might be wrong? Or is this the expected behavior? Thanks.