At the end of March (referring to 1939), that is, six months before the conclusion of the treaty signed by Molotov and Ribbentrop, the fascists began to prepare for an invasion of Poland. There was no hint yet that Moscow might be disappointed in sabotaging the UK and France with an agreement on joint actions in the event of Hitler's aggression and would decide to conclude a pact with Nazi Germany.
It's not his fault
There are facts: Stalin concluded an agreement with Germany on friendship and mutual assistance. In 1939, the Red Army followed the German army into Poland. The victory over Poland was celebrated with a joint parade of the German Army and the Red Army. 24,000 Polish prisoners of war were secretly shot by the Russians in 1940.