Why does "lowpass" give a static error when using the IIR option?

Why does a static error appear between input and output when using the following command?

lowpass(ones(1,1000),500,5000,'ImpulseResponse','iir')

By the way, the static error problem can be solved by making the following correction in "lowpass.m" at line 138:

Tw = opts.TwPercentage * WpassNormalized;

instead of:

Tw = opts.TwPercentage * (1 - WpassNormalized);