Saskatoon police are investigating an assault last night where a 25-year-old driver got pepper-sprayed by a man who flagged him over.

It happened at about 10 p.m. CST on Avenue B North by 38th Street.

Police say the attacker waved down the driver -- and then sprayed him when he rolled down his window.

Staff Sergeant Grant Obst says motive is still a mystery.

"There were some words uttered by the suspect but the victim, for whatever reason, whether it was traffic noise, whatever the reason was, couldn't hear or it it was inaudible, he couldn't understand what the suspect said," Obst said. 

The attacker then took off on foot.

The police K-9 unit tried unsuccessfully to track him down.

The victim was treated at the scene by paramedics.

The attacker is described as a white male in his mid-20s wearing a black jacket. Anyone with any information is asked to call police.