新学者向大神们求教为何函数未定义

Sub compare()
Dim i As Integer
Dim j As Integer
For i = 3 To 12
For j = 3 To 12
If Sheets("爱如生").Cells(i, 1) = Sheets("方志").Cells(j, 1) Then
If Sheets("爱如生").Cells(i, 2) = Sheets("方志").Cells(j, 2) Then
Sheets("爱如生").Cells(i, 8) = "exist"
End If
End If
Next j
nexi i
End Sub