diff error

Hi, 
xmaxima 5.10.0 for windows has the follow error:
(%i1) f:3*x[2](t)^ 2;
				       2
(%o1) 				    3 x (t)
				       2
(%i2) diff(f,x[2](t),1);
The argument to verbify must be a symbolic name:
x
 2
 -- an error.  Quitting.  To debug this try debugmode(true);
(%i3) 

Araceli