I get this error message on my program Conversion from string "" to type 'Double' is not valid.

Private Sub txtPrice_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtPrice.TextChanged

        Dim a As Double = txtPrice.Text
        Dim i As Integer
        i = 2999
        If txtPrice.Text > i Then
            MessageBox.Show("Pls insert the Special Code when price max 3k", "Information")
            Form3.Show()
        End If
    End Sub