Nyquist function returns error

I get an error when trying to use the nyquist function, either with or without an argument. The error stems from a 'switch' argument having a double as an input. I have Matlab r2010a student edition with control system toolbox version 8.5 running on a 64-bit Dell Latitude E6510 with Windows 7. Please let me know if more information would be useful. The error is as follows:

 num=[1 2];
 den=[1 -2 -3];
 h=tf(num,den)
 nyquist(h)

??? SWITCH expression must be a scalar or string constant.

Error in ==> cot at 30
switch varargin{arg}

Error in ==> freqgrid>LocalResGrid at 244
   ct = -cot(angles);

Error in ==> freqgrid at 102
[wnew,dlnew,drnew] = LocalResGrid(zp(:),Ts,dnpts,Grade);

Error in ==> ltidata.freqresp at 46
   w = freqgrid(z,p,Ts,Grade,Focus);

Error in ==> resppack.ltisource.nyquist at 24
         [mag,phase,w,FocusInfo] = freqresp(SysData(ct),1,wspec,true);

Error in ==> wavepack.waveform.draw at 21
   feval(this.DataFcn{:});

Error in ==> wrfc.plot.draw at 18
      draw(wf)

Error in ==> wrfc.plot.init_listeners>LocalRefreshPlot at 119
   draw(this)

Warning: Error occurred while evaluating listener callback. 
> In lti.nyquistplot at 103
  In lti.nyquist at 83
  In exresp at 40
  In nyquist at 67

Answer 1

[Walter Roberson]

The built-in cot() function does not have 30 lines or any switch statement.

Please use

which -all cot

to see if perhaps you have accidentally introduced your own cot.m that is overriding the MATLAB one.