I'm having a problem trying to plot a phase diagram for a plane pendulum. For some reason the function I created will not plot. if I solve for specific variable it will give me a solution but anything else I do with it will not work (Manipulate[], Plot[], etc..). Can someone help me out?

thetadot[m_, g_, l_, \[Theta]_, \[Theta]max_] := 
  2 Sqrt[(g/l)[Sin[\[Theta]max/2]^2 - Sin[\[Theta]/2]^2]];

Manipulate[
 Plot[thetadot[20, 10, 1, \[Theta], \[Theta]max], {\[Theta], -2 Pi, 
   2 Pi}], {\[Theta]max, 0, 2 Pi}]