plot2d and quad_qags feeding non-numeric input to user function

I can't use the following user-defined function in the numerical 
procedures plot2d and quad_qags because they feed it non-numeric values 
as input.  In the case of plot2d I can get around this with a discrete 
plot, but I can't find a workaround for the numeric integration. (This 
is an easy example to integrate analytically, but I am interested in the 
general case.)

bell(x):= block([dum],
      if not numberp(x) then print("bell: error, x = ",x),
      if x <= -1 then dum : 0
      else if x >= 1 then dum : 0
      else dum : (1 - x^2)^2,
      return(dum)
      ) ;

plot2d(bell, [x,-1,1]); /* OK */
trace(bell);
plot2d(bell(x), [x,-1,1]); /* input non-numerical */
qags_out: quad_qags(bell(s), s, -1, 1); /* input non-numerical */

(%i83) trace(bell);
(%o83)                              [bell]
(%i84) plot2d(bell(x), [x,-1,1]); /* input non-numerical */
1 Enter bell [x]
bell: error, x =  x
1 Exit  bell dum
COERCE-FLOAT-FUN: no such Lisp or Maxima function: dum
  -- an error. To debug this try: debugmode(true);
(%i85) qags_out: quad_qags(bell(s), s, -1, 1); /* input non-numerical */
1 Enter bell [s]
bell: error, x =  s
1 Exit  bell dum
COERCE-FLOAT-FUN: no such Lisp or Maxima function: dum
  -- an error. To debug this try: debugmode(true);

I've tried converting the input with ev(x,numer) but that doesn't work:

/* lbell(x) := bell(log(x))/x; */
lbell(x) := block(
      [xn: ev(x,numer,infeval)],
      if not numberp(xn) then print("lbell: error, xn = ",xn),
      bell(log(xn))/xn
      );

(%i34) plot2d(lbell(x),[x, 0.01, 10]);
lbell: error, xn =  x