Butter Command - Maximum Order and Cut Off Frequency - Issues

[Shadders]

Hi,
I am using the butter command as follows :

[b,a] = butter(12, 0.95);

This works. 

If i increase the order to above 64, or want a cut off at the PI frequency
(fs/2), Wc=1, with a very high order, then the filter returned seems to be
in error. Example of failure :

[b,a] = butter(12, 0.99);
[b,a] = butter(128, 0.95);

I assume i have passed the limits of the Octave capability.

Currently using Fedora 26, Octave v4.2.1.

I had a quick look at sourceforge - nothing about limits of the order or the
cut off frequency. 

Any guidance gratefully received. 

Regards,
Shadders. 



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