Hamer has yet to recover from the ankle injury he sustained in the warm-up prior to the 2-0 win over Doncaster last month so Ben Alnwick continues between the sticks.
Captain Johnnie Jackson will be hoping for a return to the starting line-up after dropping to the bench for the 1-0 midweek loss at Reading which left Charlton knowing defeat at Huish Park will see them drop into the bottom three.
Defenders Chris Solly and Leon Cort remain sidelined by knee injuries.
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