Math Operations Help

Honestly, I must be doing something silly and not seeing it but it’s driving me mad. The “Talent_Mean” Operation works perfectly - what’s wrong with “Talent_SD” (Standard Deviation)? All I get back is “{Invalid Expression}” - something must be wrong with how this has been keyed in… 

$e{ sqrt( ( ( q://QID3/SelectedAnswerRecode/1 - e://Field/Talent_Mean ) ^ 2 ) + ( q://QID3/SelectedAnswerRecode/2 - e://Field/Talent_Mean ) ^ 2 ) + ( q://QID3/SelectedAnswerRecode/3 - e://Field/Talent_Mean ) ^ 2 ) + ( q://QID3/SelectedAnswerRecode/4 - e://Field/Talent_Mean ) ^ 2 ) + ( q://QID3/SelectedAnswerRecode/5 - e://Field/Talent_Mean ) ^ 2 ) + ( q://QID3/SelectedAnswerRecode/11 - e://Field/Talent_Mean ) ^ 2 ) )  / ( ( 6 - e://Field/Talent_Mean ) ^ 2 ) ) }