### Possible inconsistency in symbolic limits

I was trying to manipulate some symbolic expressions and got an unexpected error.
Here is a minimal example,

```
u=integrate(x**3/(exp(x)-1),(x,0,oo))
```

The expression is quite messy, but the result is known. When I try to use any simplification routine, like

```
u.simplify()
```

sage displays the error message

```
RuntimeError: ECL says: Error executing code in Maxima: limit: direction must be either 'plus' or 'minus'; found: _SAGE_VAR_minus
```

As far I understood, the expression is defining lateral limits by symbolic variables, while maxima handle such limits as strings. If it is the case there is an internal conflict here. Is it the case?