Germany's foreign ministry summoned the Russian deputy ambassador to Berlin to discuss the detention of German political scientist Hans-Henning Schröder who was ultimately refused entry to the country at a Moscow airport. "The envoy was presented with the German government's formal protest over the fact that a renowned German academic was unable to enter Russia yesterday despite the fact he had been issued a visa," a foreign ministry spokeswoman said. "This is unacceptable." Schröder, 62, is the head of the research group on Russia at the German Institute for International and Security Affairs. He had been invited to a conference in Moscow organised by Russia's Higher School of Economics. According to Schröder, Russian border guards detained him at the Domodedovo airport on the grounds that he poses a “security risk” for Russia. He spent a night in a cell with illegal immigrants after which had to return to Germany. Although Schröder has the reputation of “moderate” and “balanced”, he is also known for his critical comments about Russian Prime Minister Vladimir Putin.