CHICAGO (WLS) -- An armed thief stole a large amount of cash late Tuesday night from a popular restaurant on Chicago's North Side, police said.A man with a gun entered Summer House Santa Monica in the 1900-block of North Halsted Street through the back door around 11:25 p.m., investigators said.He allegedly put a black backpack on the ground and told employees to put money in it. After workers placed cash in the bag, the suspect ran. He may have gotten away with as much as $20,000. That amount has not yet been confirmed by police.A business next door was open at the time of the robbery. The owner said the robbery happened very quietly and he was shocked to hear about it Wednesday morning.No injuries were reported. No one is in custody.