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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.