# Numerical Integral with variable params

I am trying to write some code to plot a function of an integral whose integrand depends on two variables, much like the following:

```
a = var('a')
b = var('b')
plot3d(numerical_integral((x+a+b), 0, 1, params=[a,b])[0],(a,0,1),(b,0,1))
```

This code raises an error `NotImplementedError: free variable: b`

. Which would be the correct way to implement this code?