A judge has acquitted a Calgary man of luring a teenage girl over the Internet and sexually assaulting her, because there was reasonable doubt the accused knew how young she was.

Matthew Allan Armstrong, now 28, was charged with luring, sexually assaulting and abducting a 13-year-old girl in December 2006. Police began investigating after they received a report that the girl was missing.

On Wednesday, provincial court Judge Bruce Millar found Armstrong not guilty of all five charges against him, saying the accused took reasonable steps to ascertain the complainant's age.

"She told my client and my client's roommates that she was 16," said defence lawyer Balfour Der. "She sent pictures to my client where she was naked from the waist up. She talked about having past sexual experience with other males. So, the judge took those things into account."

The judge also took into account Armstrong's disability, myotonic dystrophy, which causes muscle weakness and delayed behavioural development, said Der.

Armstrong met the girl over the internet, chatted with her online and eventually agreed to meet her in person. The court heard that on the night they met, the two had sex about five times and the girl stayed over, said Der.

Armstrong will remain in custody as he is charged in another internet luring case involving an undercover police officer. His next scheduled court date in that case is Nov. 6.

With files from Andree Lau