为什么会出现下标越界？

dim b(12) as string
For m = Asc("a") To Asc("j")
    If m <= Asc(maxchezubianhao) Then
       For j = 0 To 12
         j = (j + 1) Mod 13
                If Jend = j Then
                       Jend = lastfallindex
                       Exit For
               ElseIf Chr(m) = b(j - 1) Then
                     Fallmatrix(i, j - 1) = Chr(m)
                     lastfallindex = j - 1
               End If
       Next j
       m = m + 1
       For n = lastfallindex + 1 To 12
          If Chr(m) = b(n) Then
             Fallmatrix(i, n) = Chr(m)
          End If
       Next n
    i = i + 1
    Else
     Exit For
   End If
   m = m - 1
 Next m
那个 ElseIf Chr(m) = b(j - 1) Then    b(j-1) 为什么会出现下标越界？