Macros script Excel, filling in blanks with NULLS, error 400

All i'm trying to do is fill in NULL in the cells that are blank.

source code:

Sub fillNULLs()

Dim count As Int开发者_StackOverflow中文版eger
count = 0

Dim col As Integer
col = 2

While count < 23403

If (Cells(count, col) = "") Then

Cells(count, col) = "Null"

End If

count = count + 1

Wend

End Sub

I run the macros and i get a 400 and an okay button. Can anyone spot the issue

Sub fillNULLs()
Dim a, b As Range

Set a = Range("B1:B23402")
For Each b In a
   If (Trim(b.Value) = "") Then b = Null
Next b

End Sub  

Edit

To check it is doing what it must do

Sub fillNULLs()
Dim a, b As Range

Set a = Range("B1:B23402")
For Each b In a
   If (Trim(b.Value) = "") Then b = "Null"
Next b  

End Sub  

You will see "Null" in the modified values.

I guess that count should be 1 at first before While loop