Field Calculator VB Script Codeblock IF statement syntax is wrong?

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I can't figure out why the syntax is wrong in Field Calculator ArcMap 10. The complete codeblock is

Dim CapitalDia As NumberDim TwagDia As NumberDim X As NumberCapitalDia = [New_Pipe_S]TwagDia = [DIAMETER_1]If CapitalDia= 0 Then X = TwagDiaElse X = CapitalDiaEnd IfAnd then, the bottom window is

Final_Dia = X

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