Why didn't Britain and France declare war on the USSR when it invaded Poland in 1939?

I’m not really knowledgeable about that era in history, but I do know that Germany and Russia decided to carve up Poland. When Germany invaded Poland, Britain and France declared war. When Russia invaded Poland days later, there was no declaration of war on Russia from Britain and France. Why is that ?

This might help - http://www.auburn.edu/~sealyke/wwii/poland.html

We had another thread on this not too long ago, WW II - Why didn’t the Allies declare war on the USSR?, if you want to see what was said there.