Riis met a cross from Brad Potts flush on the volley and found the net to earn a point.
It was the second time Riis had equalised in the game, the Robins having taken the lead first through Chris Martin and then Antonie Semenyo.
Here are the player ratings from the Championship clash, with Riis pick of the bunch.
5. Ben Whiteman 6
Like many of his team-mates, Whiteman struggled in the first half. Got better in the second half and more of an influence. Photo: Mick Walker/Camerasport
6. Brad Potts 7
Worked hard down the right flank and made a long-busting run to set up the late equaliser for Emil Riis. Very energetic. Photo: Mick Walker/Camerasport
7. Alan Browne 5
The skipper had an off day in midfield and couldn’t get to grips with Massengo. Browne, though, did play his part in the second equaliser. Photo: Mick Walker/Camerasport
8. Ryan Ledson 5
The midfielder was off his game with too many passes going astray. He was substituted at the interval. Photo: Camerasport