Receiving [Expression.Error]

[thechad13]

I have received the following error when adding a Custom column: [Expression.Error] We cannot convert the value null to type Logical.

I used one column that is formatted as Whole Number and I want the custom column to display values based on the row value with the initial Whole Number column. Here is the DAX that I have used:

Table.AddColumn(#"Expanded Table Column1", "Axle Category", each if ([Total Axles]=2) then "2" 
else if ([Total Axles] = 3 or [Total Axles]=4) then "3-4" else if ([Total Axles]=5 or [Total Axles]=6 or [Total Axles]=7) then "5-7"
else if ([Total Axles]>7) then "7+"
else 0)

The new column default format is ABC123. 

Why am I getting this error and is there a way around it?

Thanks in advance. 

[HotChilli]

Do you have a null in the Total Axles column?

[thechad13]

No there isn't which is another reason why it doesn't make any sense. Even when I load everything there are none.