Why does the PBURG function in the Signal Processing Toolbox 6.

Why does the PBURG function in the Signal Processing Toolbox 6.2 (R14) throw an error message for some sampling frequencies? Why does the PBURG function in the Signal Processing Toolbox 6.2 (R14) throw an error message for some sampling frequencies?   For example: dt = 9.999999999999787e-003; pburg(1:1000, 4, 256, 1/dt) Produces the following error message: ??? Error using ==> abstractfreqresp.initialize>setdefaultsnparse Frequency vector cannot contain frequency points outside the Nyquist frequency interval, i.e., frequencies must lie within the range 0 to 50 Hz. Error in ==> pburg at 63 hpsd = dspdata.psd(Pxx,freq{:},'SpectrumType',options.range);