Searching keywords in title

Im using the below code for seacrhing a set of keywords in a cell which has title. While running the code im getting "Run Time error 13" Type mismatch on b = cell.Value line.

Application.ScreenUpdating = False
Dim col As Range, cell1 As Range, a As String, b As String, i As Integer
Set col = Range("KW[KW1]")
Dim target, cell As Range
Sheets("Data").Select
Set target = Range(Range("B1"), Range("B65536").End(xlUp))
Dim term, tag As String
    For Each cell1 In col
    a = cell1.Value

    term = a
    tag = a

        For Each cell In target
           b = cell.Value
           ' If InStr(1, " " & cell & " ", " " & term & " ", 1) Then
          If Module1.ExactWordInString(b, a) Then
                For i = 1 To 15
                    If cell.Offset(0, i).Value = "" Then
                    cell.Offset(0, i).Value = tag
                    Exit For
                    End If
                Next i

        End If

        Next cell
    Next cell1
     Application.ScreenUpdating = True

However its running perfectly if we have 1000 tiltes in a column, but i want to run this code for a massive range upto 50,000 to 200,000. Please help me.