### error: Some variable ranges specify variables while others do not

here is my code:

https://cocalc.com/share/f7766c5e-2f4d-4916-bb0d-74a7182e5fd5/E-Feld-2019-03-12-114056.sagews?viewer=share

I output the function to plot, after subs'ing values, and x and z are the only remaining variables:

```
1/4/(pi*sqrt(x^2 + z^2)*((1.46359724615010e-8)*z*heaviside(z - 0.250000000000000)*heaviside(-z + 0.750000000000000)/(76*z - 13.5625000000000) + (1.46359724615010e-8)*z*heaviside(z - 0.750000000000000)/(21.7500000000000*z + 27.1250000000000) + (6.72918274092000e-10)*heaviside(-z + 0.250000000000000)))
```

but when i try to contour_plot this it gives me the error in the title.

the plot command is this:

```
contour_plot(pic,(x,-0,5,1.5),(z,-1,2))
```

what can I do to specify more variables?I googled the error message and found it in the documentation, but there it is without explanation how to fix it.