[Newbie] Plot problem

If I execute the following:

  NUMER:TRUE;
  REALONLY:TRUE;
  f(x,y):=1/(x^4+y^4);
  F(x):=quad_qagi(f(x,y),y,0,both)[1];

Then querying F(1); F(2); etc outputs sensible numbers.
However, trying to get a graph of F:

  plot2d(F(x),[x,-2,2]);

fails with
 
  Error in FUNCALL [or a callee]: ((MEXPT SIMP)
                                  ((MPLUS SIMP) 1.0
                                   ((MEXPT SIMP) |$x| 4))
                                  -1) is not of type (OR RATIONAL
                                                      LISP:FLOAT).

I have a vague idea the solution involves dropping a "'" in the right
place, or inserting some MODE_DECLAREs or using FLOATDEFUNK.  However,
I've completely failed to get anywhere with either of them.  What's the
trick ?

[I'm using maxima 5.9.1 on Debian Sarge.  Things like:
plot2d(sin(x),[x,-%pi,%pi]);
work fine, coming up in gnuplot.]

Thanks for any help
Tim