one bug about reghdfe command

[Hualin Chl]

i use the reghdfe command "reghdfe y did $cv4 $firmcv , ab(stkcd year) vce(cl city)" ,then the 17.0 stata show me "
(dropped 393 singleton observations)
_assert_abort(): 3498 stdevs are missing; is N==1?
assert_msg(): - function returned error
compute_stdevs(): - function returned error
standardize_data(): - function returned error
FixedEffects::partial_out(): - function returned error
<istmt>: - function returned error
r(3498);"

I am quiet confused by above bug.so why does this bug show up and how can i solve this bug?