Error 438 on Pivot calculated field creation using string variables

Hello,

I am trying to drive the creation of a large number of fields without hardcoding everything.
.CalcuatedFields.Add works ok when Const strings are used but fails (438) when variables are used.



Sub TEST()

Dim pt As PivotTable

Dim pf As PivotField

Dim calcfld_1_src As String
Dim calcfld_1_formula As String
Dim calcfld_1_name As String

calcfld_1_src = "JAN APPR"
calcfld_1_formula = "= Jan Billable Hours + Jan Cost Recovery Hours "


Sheets("Pivot").Activate
Set pt = ActiveSheet.PivotTables(1)


With pt

For Each pf In pt.CalculatedFields

pf.Delete

Next pf

.CalcuatedFields.Add calcfld_1_src, calcfld_1_formula

' ......................



When calcfld_1_src, calcfld_1_formula are defined as constants :
Const calcfld_1_src As String = "JAN APPR"


the calculated field is created.

Is the there a way around this?

Best regards
Stephen