Is it offensive?

[joshthewriter.livejournal.com]

Здрасте всо!  I have been a long time lurker in this community and I wanted to pose a question to the group.  In my job and around my town I run into lots of people from countries like Ukraine, Belarus, and other former Soviet countries.  A long time ago I tried to speak in Russian to a woman from the Ukraine, and she became very offended because I had assumed she spoke Russian, which she did, but somehow this made her very angry.

Ever since then even though I'm conversationally adequate in Russian, and sometimes at my job it would be a lot easier if I spoke russian to my customers who don't speak english very well, I hesitate and usually just stick to english because I don't want to offend anyone like that again, especially if they're not actually from Russia.  Have any of you experienced this and do you think it's okay for me to try and speak to people in russian even though it might not be their home country's primary language?  I helped a gentleman from Belarus today with his computer and it was very difficult to communicate with him in english but I was so terrified of making him angry that I didn't even try russian, and now I feel bad because maybe I could have helped him better =/