# Sum using coefficients of a expansion

I'm trying to sum over the coefficients of an expansion but the sum gives zero. I use the taylor method to expand a function f:

```
f=sin(x)
ft=f.taylor(x,0,6)
```

then I need to use the coefficients in a sum, but if I for example do the following:

```
sum(x^(i)*ft.coefficient(x,i),i,0,5)
```

I get zero. Any idea why this is happening?