Avoid warnings in calculation of invalid variances.
Avoid warnings in calculation of invalid variances.

saimn/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,aleksandr-bakanov/astropy,mhvk/astropy,larrybradley/astropy,saimn/astropy,StuartLittlefair/astropy,mhvk/astropy,StuartLittlefair/astropy,astropy/astropy,lpsinger/astropy,larrybradley/astropy,pllim/astropy,astropy/astropy,mhvk/astropy,lpsinger/astropy,StuartLittlefair/astropy,pllim/astropy,astropy/astropy,saimn/astropy,lpsinger/astropy,pllim/astropy,larrybradley/astropy,lpsinger/astropy,mhvk/astropy,StuartLittlefair/astropy,larrybradley/astropy,saimn/astropy,lpsinger/astropy,astropy/astropy,mhvk/astropy,pllim/astropy,pllim/astropy,aleksandr-bakanov/astropy,astropy/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,saimn/astropy