Substring with patindex for substrings' length not working to extract part of a string

Question

I have a table with strings (ItemCode) like:
99XXX123456-789
12ABC221122-987BA1

They are always of a length of 11 characters (upto the - of which they always contain only one), after the - length is variable. I would like to get the part after the first 5 characters upto the - , like this.
123456
221122

I tried with substring and patindex:

SELECT SUBSTRING( ItemCode, 6, PATINDEX('%[-]%', ItemCode) - 6 ),
       PATINDEX('%[-]%', ItemCode),
       ItemCode 
FROM TableName
WHERE LEFT(ItemCode, 5) = '99XXX'

Patindex itself returns the correct value (12) but with PATINDEX('%[-]%', ItemCode) - 6 /sql should understand this as 12 - 6 = 6 / SQL Server 2012 gives an error. I could use 6 as a fix value in the patindex for the length, of course but I want to understand the reason for the error.

Answer 1

But when I am using the same query I am not getting the error:

create table #temp
(
num varchar(50)
)

--insert into #temp  values ('99XXX123456-789')
--insert into #temp  values ('12ABC221122-987BA1')

SELECT SUBSTRING( num, 6, PATINDEX('%[-]%', num) - 6 ),
       PATINDEX('%[-]%', num),
       num 
FROM #temp WHERE LEFT(num, 5) = '99XXX'