Sometimes verbs have two proper past-tense forms, which is the case with the verb spell. Which form you should use depends on where you live.
Robbie on Twitter said he and his colleagues at work had an argument about the past tense of the word spell yesterday. Is it spelt or spelled?
No need to argue; they're both allowed!
Spelled is by far the most common past tense form in America, and spelt used to be the most common past tense form in Britain and still gives spelled a run for its money. (See how the use of spelled and spelt has changed over time in British books.)