Convert SQLite Fetchone() Result to float for Math

[Extra]

So I can do this:

                        print('Selcted Item Quantity: ', SelectedItemQuantity)

                        x = float(SelectedItemQuantity)
                        y = SellPrice[0]
                        z = float(y)
                        print('Float Value = ', x)
                        print('x*z= ', x*z)
                        MaterialUsedPrice2 = x*z

                        #Calculate the Price of Material Used
                        # MaterialUsedPrice = SelectedItemQuantity * SellPrice
                        print('The Total Price for the ', '(',x,')', SelectedItemName, ' Used is: $', MaterialUsedPrice2)

And I get:

Output:
Selcted Item Quantity:  2
Float Value =  2.0
x*z=  3.0
The Total Price for the  ( 2.0 ) 1/2" PVC Coupling  Used is: $ 3.0

Which is what I want,
But then I get this error and the program crashes:

Error:
    y = SellPrice[0]
TypeError: 'NoneType' object is not subscriptable

Any way to get around/fix this?