# What does "Runtime Error: ECL says: ... is not of type FIXNUM" mean and how to fix it?

I am trying to integrate $x(0.6x^{0.5}+0.6)$ from $0$ to 1.

Here's what I tried:

```
var('x')
(x*(0.6*x^0.5+0.6)).integrate(x,0,1)
```

But I received the following error:

```
RuntimeError: ECL says: 3.0 is not of type FIXNUM.
```

What does the error mean (I am especially curious where the 3 came from) and what can I do to correctly evaluate the definite integral? According to Wolfram Alpha, the answer is 0.54.