In 1969, the plaintiff filed a complaint claiming that he was deprived of constitutional rights during an arrest. The district court dismissed the complaint for failure to state a claim. Specifically, the court made its decision based on the plaintiff’s failure to allege a lack of probable cause for the arrest. The court did not specify that the dismissal was without prejudice. In 1970, plaintiff filed this complaint, which is identical to the 1969 complaint, except that the plaintiff added a claim that the arrest was made without probable cause. The district court dismissed the complaint on the ground that the matter was res judicata on account of the 1969 dismissal. The plaintiff appealed.