The SUBSTRING function has the following syntax:

SUBSTRING ( expression, start, length )

Notes:

1. It's an error to have a NULL expression
2. The start is 1 based

To get around the NULL expression scenario, use the CASE WHEN ... expression in TSQL.

References:

https://docs.microsoft.com/en-us/sql/t-sql/language-elements/case-transact-sql

https://docs.microsoft.com/en-us/sql/t-sql/functions/substring-transact-sql