integration with infinity limit has problems

[webmasterpdx]

I've heard that this problem is common with the hp Prime, and people don't have this problem with the TI-NSPIRE or the TI89 for that matter.... So, this was a fairly simple integration problem. It shouldn't have required anything complicated.
Is there something I can tell it before giving it the integral to calculate to avoid this message, like something inside the assume() function? How can I tell the system that the function is continuous or whatever it needs to know to do the calculation. I mean if I were to do this manually, I wouldn't need anything like this.....the calculator should be able to do the same thing as me doing it manually.
If you know how, please give me the steps to type if I wanted to simply calculate
int(e^(-a*x),x,0,infinity)