Jason got wide eyed as he saw the snake lunge at Sara. He knew he wasn't going to be quick enough to stop it. 

However, Sara wasn't alone. The construct jumped and tackled the serpent and tried to pin it in place knowing it would be difficult to hold a serpent. Jason quickly tried to cast another spell on the serpent as it was being blocked by the construct. Jason cast the spell deep slumber to try and put the serpent to sleep. Even if the construct was caught by the spell as well it wouldn't sleep. It was a construct and technically couldn't sleep anyway.