According to the RIA Novosti web site, "Russia will not permit the first U.S. commercial spacecraft to dock with the International Space Station (ISS) unless its safety is fully tested, a high-ranking Russian space official said on Friday."
"We will not issue docking permission unless the necessary level of reliability and safety [of the spacecraft] is proven. So far we have no proof that those spacecraft duly comply with the accepted norms of spaceflight safety," said Alexei Krasnov, who heads the manned spaceflight department of Russia's space agency Roscosmos.
He said that to authorize docking, the Russian space agency will need to study data about the demonstration flight due to take place later this year.
"We have received no requests from NASA on the issue," the space official said.
"This plan is realistic, and ultimately commercial spacecraft will be able to dock with the space station," he said. "But all in good time."
Just my speculation, but perhaps Roscosmos is as afraid of the commercial space competition posed by SpaceX as is China.