Callum Ainley's late finish secured League Two Crewe a surprise 2-1 comeback win over Rotherham.
The tie burst into life after Rotherham edged ahead in the 21st minute through a classy finish from Will Vaulks, who curled a right-footer into the top corner.
And Rotherham were celebrating what they thought was their second goal when Kieffer Moore headed home Joe Newell's inswinging corner on the stroke of half-time. But an eagle-eyed official spotted an infringement before play had started and ordered Newell to retake the kick.
Crewe received some more unexpected assistance when they squared the tie straight after the break. Millers goalkeeper Richard O'Donnell was caught off his line as George Cooper drifted across a free-kick and Walker's flicked header found an empty net.
Vaulks went close to adding to his strike on several occasions, including flicking the crossbar with a rasping drive, while Dave Richards did well to keep out a bullet header from Millers leading scorer Moore.
But it was youngster Ainley who won it for Crewe in the 89th minute when he got on the end of a ball over the top and flicked over O'Donnell, and despite the desperate efforts of Semi Ajayi the ball carried over the line.