Strange output from unchanged generate.py

[summerstay]

When I run generate.py with no changes (except removing pdb) this is the output I see:

The pre-trained model you are loading is a cased model but you have not set `do_lower_case` to False. We are setting `do_lower_case=False` for you but you may want to check this behavior.
Decoding strategy sequential, argmax at each step
Iteration 0: this is a sentence .
	BERT prediction: . is a . .
Iteration 1: . is a sentence .
	BERT prediction: . is a . .
Iteration 2: . is a sentence .
	BERT prediction: . is a . .
Iteration 3: . is a sentence .
	BERT prediction: . is a . .
Iteration 4: . is a . .
	BERT prediction: . . a . .
Final: . is a . .

That doesn't seem to be the desired result. Why so many periods? If this is expected, can you give me an example of an input and configuration that will find the correct answer?