Modules & VBA :: 2nd Run Of A Function Calculates Wrong Results?

i am using the following function in a query:

Code:
Public Function Progress(ByVal varCompany As Variant _
, ByVal varPercent As Variant) As Variant
Static varCompanyOld As Variant
Static varValueOld As Variant
Dim Value As Variant
If varCompany & vbNullChar = varCompanyOld Then
Value = (1 + varPercent) * varValueOld
varValueOld = Value

[code]....

This function is working if I use it with two or more companies.If I have only one company, it works at the first (query) run, too.But with the same company, a second (query) run calculates different results.

I think it is because of this condition:

Code:
If varCompany & vbNullChar = varCompanyOld Then ...

If the company name is e.g. Microsoft

At the first run of the query:

Microsoft & vbNullChar = "" -> wrong

The Else calculation will be carried out.

-> That is right for the first datapoint of a company

At a second run of the query:

varCompanyOld is saved as Microsoft & vbNullChar, so:

Microsoft & vbNullChar = Microsoft & vbNullChar -> True

The main calculation will be carried out.

-> That is wrong for the first datapoint of a company