title: Excel VBA Custom Function giving "Reference Not Valid" Error
body: <p>I've attempted to track down and solve this error for hours now and I just can't figure it out. Here's the setup.</p>
  
  <p>I have one excel workbook which has two sheets in it: "Input" and "Calculations". I've written in a few custom functions to calculate certain things. These functions are used in the "Calculations" sheet, but reference cells in the "Input" sheet. Now if I'm just using the sheet itself everything works perfectly fine and the functions work.</p>
  
  <p>However, I have a second excel workbook which interacts with the first. I have a macro in the second workbook which attempts to define values in the "Input" sheet of the first workbook. However, when it does this, suddenly the functions don't work. When I attempt to trace the error of the cell in the "Calculations" sheet, and attempt to go to the cell in "Input" sheet, it claims the reference is invalid.</p>
  
  <p>I have no idea what the problem is. At first I thought it maybe had something to do with the name of the first workbook (which was "Log K Calculator 7.0.0.xlsm") but I've tried changing that and I get the same problem. Here is the macro in the second sheet which attempts to change the values in the first:</p>
  
  <pre><code>Sub macro()
  
  Dim logK As String
  Dim this As String
  logK = "Log K Calculator 7.0.0.xlsm"
  this = ThisWorkbook.Name
  
  Workbooks(logK).Activate
  
  Workbooks(logK).Sheets("Input").Cells(11, 4).Value = Workbooks(this).Sheets(1).Cells(1,   "B").Value
  Workbooks(logK).Sheets("Input").Cells(12, 4).Value = Workbooks(this).Sheets(1).Cells(2, "B").Value
  Workbooks(logK).Sheets("Input").Cells(14, 4).Value = Workbooks(this).Sheets(1).Cells(3, "B").Value
  
  End Sub
  </code></pre>