Replace() -- Syntax Error

Why doesnt':
--Dim currname As String
--currname = ActiveDocument.FullName
--replace(currname," ","%20",1,-1,1)
work? (at least in Word 2003's implementation of VBA)? I get, "Compile Error. Expected =" I think I am faithfully following the syntax described in the Word/VB help file (see end of this posting), but obviously I am not if I get an error message. My goal is to create a string variable equal to the complete filename but with any spaces replaced with "%20" (minus the quotes). The result will be a click-able hyperlink when pasted into an email.



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FROM WORD/VBA HELP FILE

Replace Function
  Description -- Returns a string in which a specified substring has been replaced with another substring a specified number of times.
  Syntax -- Replace(expression, find, replace[, start[, count[, compare]]])
  Where:
   -- expression = String expression containing substring to replace.
   -- find = Substring being searched for.
   -- replace = Replacement substring.
   -- start (Optional) =  Position within expression where substring search is to begin. If omitted, 1 is assumed.
   -- count (Optional) =  Number of substring substitutions to perform. If omitted, the default value is –1, which means make all possible substitutions.
   -- compare (Optional) =  Numeric value indicating the kind of comparison to use when evaluating substrings where:
    == vbUseCompareOption –1 Performs a comparison using the setting of the Option Compare statement.
    == vbBinaryCompare 0 Performs a binary comparison.
    == vbTextCompare 1 Performs a textual comparison.
    == vbDatabaseCompare 2 Microsoft Access only. Performs a comparison based on information in your database.