I have been trying to obtain a 3D plot of f9x,y)=(x+y)e^(-x^2-y^2) and it keeps outputting  "warning unable to find numerical values for the in the region....blah blah blah"

Here is my call sequence

> plot3d((x+y)*e^(-x^2-y^2), x = -1.7 .. 1.7, y = -1.7 .. 1.7);
Warning, unable to evaluate the function to numeric values in the region; see the plotting command's help page to ensure the calling sequence is correct

Anyone know what to do? ...