Former Reading and Hull marksman Nicky Forster gave the hosts the lead with a soft penalty in the 10th minute.
But Lambert, after twice being denied by the woodwork, gave Rovers their first point of the season from the spot after Craig Disley had his shirt pulled inside the area.
Forster should have hit the target in the fifth minute when keeper Steve Phillips saved his low shot with his legs after Dean Cox had provided the opening.
But Forster broke the deadlock from the spot after Rovers midfielder David Pipe was harshly penalised for impeding the striker.
Rovers came close to equalising a minute after the break when Lambert bent a free kick around the wall and rattled the crossbar with keeper Michel Kuipers beaten. Lambert was unfortunate again 14 minutes from time when his low shot from 12 yards cannoned back off the post.