How to add statement numeric value <> null value "" and show Run-time error '13 Type mismatch VBA

there was an error of mismatch type, so the problem on

ElseIf CInt(Cells(2, 3)) <> Year(Sheet9.Cells(a, 51)) And Sheet9.Cells(a, 15) = "APP" Then

I think the problem is not the CInt, CInt function is supplied with a text string. In my opinion the problem code not read <> Year(Sheet9.Cells(a, 51)) because reflect value "" I don't fill any value on InputBox which mean "" and my question, how to add statement elseif number <> blank value ("")?

here my full code:

Private Sub btn_check_Click()

Application.ScreenUpdating = False
Dim a, b, x, y, z, c As Long
Dim indicator, puzzlePartner, puzzleRaw As String

a = 5
b = 2
c = 5

Do Until Cells(c, 1) = ""
     Sheet9.Cells(c, 3).Select
        Sheet9.Cells(c, 56) = Trim(CStr(Left(Sheet9.Cells(c, 3), 15)))
    c = c + 1
    Loop

a = 5

Do Until Sheet9.Cells(a, 1) = ""
    Do Until Sheet4.Cells(b, 104) = ""


    If a = 2000 Then
    c = 1
    End If

    If Sheet4.Cells(b, 9) = Sheet9.Cells(a, 9) And Sheet4.Cells(b, 27) = Sheet9.Cells(a, 27) And Sheet4.Cells(b, 26) = Sheet9.Cells(a, 26) And _
         Sheet4.Cells(b, 25) = Sheet9.Cells(a, 25) And Sheet4.Cells(b, 24) = Sheet9.Cells(a, 24) And Sheet9.Cells(a, 52) = Sheet4.Cells(b, 104) And _
         LCase(Trim(Sheet4.Cells(b, 21))) = LCase(Trim(Sheet9.Cells(a, 21))) Then
         Sheet9.Cells(a, 53) = "True"
        GoTo 1
    ElseIf CInt(Cells(2, 3)) <> Year(Sheet9.Cells(a, 9)) And Sheet9.Cells(a, 15) = "Withdrawn" Then
        Sheet9.Cells(a, 53) = "True"
        GoTo 1
    ElseIf CInt(Cells(2, 3)) <> Year(Sheet9.Cells(a, 9)) And Sheet9.Cells(a, 15) = "DC" Then
        Sheet9.Cells(a, 53) = "True"
        GoTo 1
    ElseIf CInt(Cells(2, 3)) <> Year(Sheet9.Cells(a, 9)) And Sheet9.Cells(a, 15) = "PO" Then
        Sheet9.Cells(a, 53) = "True"
        GoTo 1
    ElseIf Right(Sheet9.Cells(a, 7), 3) = "(S)" Then
        Sheet9.Cells(a, 53) = "True"
        GoTo 1
    ElseIf CInt(Cells(2, 3)) <> Year(Sheet9.Cells(a, 51)) And Sheet9.Cells(a, 15) = "APP" Then
        Sheet9.Cells(a, 53) = "True"
        GoTo 1
        ElseIf CInt(Cells(2, 3)) <> Year(Cells(a, 12)) And Right(Cells(a, 52), 3) = "RNW" Then
            Sheet9.Cells(a, 53) = "True"
            GoTo 1
            End If
        End If
2:
    b = b + 1
    Loop
1:

    b = 2
a = a + 1
Loop

End Sub

Anyone have idea why is this happening?