Passing Null

This allows these functions to handle passing in the Null keyword.
LEFT - Returns a Variant data type and can handle Null.
LEFT$ - Returns a String data type and cannot handle Null.

The functions that return a String data type will generate an an error if you call them with a value that is Null.
The functions that return a Variant data type can handle Null values without an error.
The functions that do not include a dollar sign can handle Null.
The functions that include a dollar sign ($) cannot handle Null and can only accept Strings.
If you want to be able to handle passing Null into the LEFT function you must pass the results to a Variant data type.

Dim vVariant1 As Variant 
Dim vVariant2 As Variant 
vVariant1 = Left(Null, 3)    'returns Null  
vVariant2 = Left$(Null, 3)   'ERROR invalid use of Null  

You should try and avoid using the Variant data type though, unless there is no alternative.
A better approach would be to pass the results into a String data type and prevent a Null argument altogether.