?truncate error when plotting



When I use ?truncate in a function, and then try to plot the function I 
get an error, e.g.

f(t):=?truncate(t)
plot2d([f(t)],[t,-5.0,5.0])$
Maxima encountered a Lisp error:
 Error in MACSYMA-TOP-LEVEL [or a callee]: $T is not of type (OR
                                                              RATIONAL
                                                              LISP:FLOAT).
Automatically continuing.
To reenable the Lisp debugger set *debugger-hook* to nil.

Maxima encountered a Lisp error:
 Error in MACSYMA-TOP-LEVEL [or a callee]: $T is not of type (OR
                                                              RATIONAL
                                                              LISP:FLOAT).
Automatically continuing.
To reenable the Lisp debugger set *debugger-hook* to nil.

Since my use of Maxima to date has been limited, my error may be very 
silly, but would appreciate some help.
Thank you,

John