plot problems



Hi to everybody,

while 3d-plotting a simple function the following output appears:

(C21)   g5(z1,z2):=(MODE_DECLARE(z1,FLOAT,z2,FLOAT),(IF (z2 < (3-1)) THEN (IF (z1 < (3-1)) THEN 0.5 ELSE (-(3-z1)^2*0.5 +(3-z1))) ELSE (IF (z2 > z1) THEN (3-z2)^2*0.5 ELSE z1*z2-3*z2-z1^2*0.5+3^2*0.5)))$
(C22) compile(g5)$
(C23)  plot3d(ev(g5(z1,z2),noeval),[z1,0,3],[z2,0,3])$

above everything was OK with plot3d, but below the only difference is
single replacement of 0.5 with 1/2 and this causes the error message.

(C24)   g6(z1,z2):=(MODE_DECLARE(z1,FLOAT,z2,FLOAT),(IF (z2 < (3-1)) THEN (IF (z1 < (3-1)) THEN 1/2 ELSE (-(3-z1)^2*0.5 +(3-z1))) ELSE (IF (z2 > z1) THEN (3-z2)^2*0.5 ELSE z1*z2-3*z2-z1^2*0.5+3^2*0.5)))$
(C25) compile(g6)$
(C26)  plot3d(ev(g6(z1,z2),noeval),[z1,0,3],[z2,0,3])$

*** - argument to COMMON-LISP:FLOAT should be a real number: ((RAT SIMP) 1 2)
The following restarts are available:
R1 = Macsyma top-level

1. Break [2]> 

Any suggestions how to avoid this?

Thanks,
--
Alexander