Karl Robinson's men were held to a goalless draw by minnows Cambridge City on Friday night, while AFC Wimbledon drew 1-1 at York on Saturday.
Carlisle have once again be drawn at home in the FA Cup with Bournemouth due to visit Brunton Park in the second round.
The Cumbrians needed two late goals to defeat non-league outfit Ebbsfleet 4-2 in the first round.
Port Vale manager Micky Adams will return to Bramall Lane when his side meet Sheffield United in the second round of the FA Cup
Adams was at the helm when the Blades were relegated from the Championship two seasons ago, and will head back to south Yorkshire after Vale defeated Forest Green 3-2 in the first round.
The winner of the replay involving Morecambe and Rochdale will travel to Coventry in the second round of the FA Cup.
The Sky Blues were comfortable 3-0 winners against minnows Arlesey on Saturday, while the Shrimps and Dale played out a 1-1 draw at the Globe Arena.
Southend face a trip to League One strugglers Bury in the second round of the FA Cup.
Paul Sturrock's side booked their place in the second round with a comfortable 3-0 home triumph over Stockport.
FA Cup second round draw:
Preston North End v Gillingham
Bury v Southend United
Sheffield United v Port Vale
Carlisle United v AFC Bournemouth
Crewe Alexandra v Burton Albion or Altrincham
Northampton Town or Bradford City v Brentford
Luton Town or Nuneaton Town v Dorchester Town
Alfreton Town v Gloucester City or Leyton Orient
Oldham Athletic v Doncaster Rovers
Braintree Town or Tranmere Rovers v Chesterfield
Rotherham United v Notts County
Cheltenham Town v Hereford United
Guiseley or Barrow v Macclesfield Town
Accrington Stanley v Oxford United
Lincoln City or Walsall v Mansfield Town or Slough Town
Harrogate Town v Hastings United
Coventry City v Morecambe or Rochdale
Cambridge City or MK Dons v York City or AFC Wimbledon
Crawley Town v Chelmsford City
Fleetwood Town v Aldershot Town