Question: cannot change horizontal range on plot

Hi all I have > tst:=-(1-cos(p))/p^2+1/2*cos(1/4*p)^2*cos(1/8*p)^2*cos(1/16*p)^2: >plot(tst); which seems to work OK. However, this generates an error plot(tst,Pi..2*Pi); Warning, unable to evaluate the function to numeric values in the region; see the plotting command's help page to ensure the calling sequence is correct Plotting error, empty plot ...as does any other range I have tried. I guess it's something very simple, but if someone can point me in the right direction I would appreciate it !
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