"ByRef argument type mismtach" on string variable *sometimes*
 

I am using the code at http://www.vbaexpress.com/kb/archive.php/k-743.html
to change the color of some text in an Excel 2003 spreadsheet. I am
using the following type of invocation:

Dim TargetString, OtherString As String
TargetString = "QuickBrownFox"
Call xlCellTextMgmt( SomeCell, _
TargetString , , , , , ColorIndexRed )

This generates the error in the subject line. However, this
invocation is fine:

Call xlCellTextMgmt( SomeCell, _
"QuickBrownFox", , , , , ColorIndexRed )

This invocation is fine, too:

OtherString = "QuickBrownFox"
SomeCell = OtherString
Call xlCellTextMgmt( SomeCell, _
OtherString, , , , , ColorIndexRed )

What could possibly cause the first invocation to give the subject
error?