Invalid column name despite checking for it

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SQL

Hello,

Im trying to check if a field (isParent) exists and if it doesn't, print a message and stop. If it exists, continue based one one of two choices. However, when I know the field doesnt exist, it throws an invalid field error on the first instance of isParent is Null. Why is it doing this?

Thanks

declare @RUNTYPE varchar(20);
--set @RUNTYPE = 'Incremental Run'
set @RUNTYPE = 'Full Run';

--select COL_LENGTH('eddsdbo.document','isParent')


IF COL_LENGTH('eddsdbo.document','isParent') is NULL
	PRINT 'isParent field is missing. Please create it and retry.'
ELSE 
	BEGIN
		IF @RunType = 'Incremental Run' --AND COL_LENGTH('eddsdbo.document','isParent') is not null
			BEGIN
			--populate yes for parent docs and run first
			update EDDSDBO.DOCUMENT
			set isParent = 1
			where ((BEGATTACH is NULL) or (BEGDOC = BEGATTACH))
			and isParent is NULL
					
			--remaining docs with isParent = NULL should be set to No at this point
			update EDDSDBO.DOCUMENT
			set isParent = 0
			where isParent is NULL

			PRINT 'Incremental run successful!'
			END

		ELSE --IF @RunType = 'Full Run'
			BEGIN
			update EDDSDBO.DOCUMENT set isParent = NULL

			--populate yes for parent docs and run first
			update EDDSDBO.DOCUMENT
			set isParent = 1
			where ((BEGATTACH is NULL) or (BEGDOC = BEGATTACH))
			and isParent is NULL
					
			--remaining docs with isParent = NULL should be set to No at this point
			update EDDSDBO.DOCUMENT
			set isParent = 0
			where isParent is NULL

			PRINT 'Full run successful!'
			END
	END