This document discloses a method, apparatus, and computer program for automatically detecting unallowed continuation of a communication protocol procedure in a communication device. The method comprises in a test tool: marking an invalid input applied to the communication device in a sequence of operations of the communication protocol procedure; marking, with a sequence marker, a location that should not be reached in the sequence of operations of the communication protocol procedure as a result of the invalid input; and upon detecting that the communication protocol procedure has reached the location marked with the sequence marker, outputting an indication that the communication device operates in an unallowed manner.