Put berries and sugar into a saucepan, and gently cook until boiling, stirring constantly to prevent burning. Once it boils, take it off the stove. Pour the berries through a sieve to remove the seeds. Let juice sit a bit to cool off so that you can put the cornstarch in it without any problems.
Pour the juice back into the pan, add the cornstarch, and mix well. Put it back onto the stove, bring gently to a boil, stirring constantly to keep the bottom mixed up (can't let it settle!) Boil one minute to set the cornstarch. Remove from the stove, add the cinnamon, and mix well.
Then pour it into the pie shell, set it aside to cool down, then cover with plastic and put into the refrigerator. It should set up within three or four hours. Serve with whipped cream.