Why am I getting an error when using the RLOCUS function to find the root locus of my system when using the Control System Toolbox 5.1 (R12.1)?

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Why am I getting an error when using the RLOCUS function to find the root locus of my system when using the Control System Toolbox 5.1 (R12.1)?
Here is my system:
nume=[1 0 1];
dene=conv([1 0],[1 0 4]);
rlocus(nume,dene);
I get the following error upon running the above code
??? Error using ==> eig
NaN or Inf prevents convergence.
RLOCUS works fine with other systems.

Accepted Answer

MathWorks Support Team
MathWorks Support Team on 27 Jun 2009
This is a bug in RLOCUS in the Control System Toolbox 5.1 (R12.1) that has been fixed in the Control System Toolbox 5.2 (R13).
As a workaround for R12.1, please explicitly specify the Gain value in the RLOCUS command.
nume=[1 0 1];
dene=conv([1 0],[1 0 4]);
sys = tf(nume,dene);
GainK=0;
rlocus(sys,GainK);

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