# Integration of Gamma PDF fails

I'm new to Sage and, as a simple example, I'm trying to integrate the (unnormalised) gamma distribution pdf:

```
f(x,a,b) = x^(a-1)*ℯ^(-b*x)
assume(b>0,'real', a>0,'real' ,x>0,'real')
fint = f.integrate(x,0,infinity)
```

This throws

```
ValueError: Computation failed since Maxima requested additional constraints; using the 'assume' command before evaluation *may* help (example of legal syntax is 'assume(a>0)', see `assume?` for more details)
Is a an integer?
```

What am I doing wrong here?