AFC Wimbledon goalkeeper Seb Brown was central to his side frustrating League One Scunthorpe.
Chris Dagnall rounded the Dons keeper early on but could not find the finish, while Rashid Yussuff forced a save from Iron keeper Sam Slocombe.
Max Porter went close for the League Two side after a neat turn and shot in the area.
Brown made fine saves to deny Nicky Ajose's fiercely-struck effort and Mark Duffy's powerful shot.
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AFC Wimbledon boss Terry Brown told BBC London 94.9:
"It was a very, very good Scunthorpe side, probably the best footballing side we've played this year.
"We struggled to compete with them in the first half.
"We hung in there and that's what the FA Cup is about."
Scunthorpe United assistant manager Chris Brass told BBC Radio Humberside:
"We knew it was going to be a tough game because they're a well organised side. We watched them in a number of games and they're a hard-working team.
"It's a tough place to come to. I thought we started extremely well and unfortunately just couldn't get any kind of joy.
"We're still in it. We're frustrated that we've not taken a chance. However it's a clean sheet and considering we've had three changes in the backline it's pleasing."