Question: Error, (in plot) unknown optional arguments [true]

Hello. I tried to plot simple functions in MAPLE 16 and received this message:

Error, (in plot) unknown optional arguments [true]. I am not sure what the problem is and would appreciate any help. I have included an example below. Thanks.

 

> with(linalg); with(plots);
> plot(cos((1/2)*x)+sin(2*x), x = 0 .. 4*Pi);
Error, (in plot) unknown optional arguments [true]
>

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