UDF #value! Error When VBA Runs

I have written a number of UDFs, which work fine in my spreadsheet. Two of these are more complex, and I'm having a few problems...

When I run any VBA routine (i.e. control passed to VB), these two UDFs return #VALUE! errors, but I can't see why. As these are Functions, I can't think of a way to test where the error is being generated, like I would in a Sub

When I return to Excel, any recalculation fixes the problem, which makes me think something is declared incorrectly. All named cell references relate to other VBA functions, calling the relevant row / column numbers, and I think these are all correct. I've tried removing AS INTEGER etc, to avoid type mismatches, and adding APPLICATION in front of worksheetfunction...

Function precedentStart(Target As Range) As Integer
Application.Volatile
With ThisWorkbook.Sheets("sheet1")
Dim splitPrecedent As Variant, lookupI As Integer, rowI As Integer

precedentStart = .Cells(Target.Row, colOrigStart).Value
splitPrecedent = Split(Target.Text, ",")

For i = LBound(splitPrecedent) To UBound(splitPrecedent)
rowI = WorksheetFunction.Match(splitPrecedent(i), .Range("schedRefs"), 0)
lookupI = .Cells(rowI, colAdjEnd).Value
If lookupI > precedentStart Then precedentStart = lookupI
Next i
End With
End Function