Cannot insert NULL value into column error - triggers

Question

I have a issue where I want to update a column in a table and with a trigger to update same column but in another table. It says I cannot insert NULL but I can't seem to understand from where it gets that NULL value. This is the trigger:
CREATE TRIGGER Custom_WF_Update_WF_DefinitionSteps_DefinitionId ON WF.Definition
AFTER UPDATE AS BEGIN
IF UPDATE(DefinitionId)
IF TRIGGER_NESTLEVEL() < 2
BEGIN
ALTER TABLE WF.DefinitionSteps NOCHECK CONSTRAINT ALL
UPDATE WF.DefinitionSteps
SET DefinitionId =
(SELECT i.DefinitionId
FROM inserted i,
deleted d
WHERE WF.DefinitionSteps.DefinitionId = d.DefinitionId
AND i.oldPkCol = d.DefinitionId)
WHERE WF.DefinitionSteps.DefinitionId IN
(SELECT DefinitionId FROM deleted)
ALTER TABLE WF.DefinitionSteps CHECK CONSTRAINT ALL
END
END
This update statement works just fine:
UPDATE [CCHMergeIntermediate].[WF].[Definition]
SET DefinitionId = source.DefinitionId + 445
FROM [CCHMergeIntermediate].[WF].[Definition] source
But this one fails:
UPDATE [CCHMergeIntermediate].[WF].[Definition]
SET DefinitionId = target.DefinitionId
FROM [CCHMergeIntermediate].[WF].[Definition] source
INNER JOIN [centralq3].[WF].[Definition] target
ON (((source.Name = target.Name) OR (source.Name IS NULL AND target.Name IS NULL)))
I get the following error:
Msg 515, Level 16, State 2, Procedure Custom_WF_Update_WF_DefinitionSteps_DefinitionId, Line 7
Cannot insert the value NULL into column 'DefinitionId', table 'CCHMergeIntermediate.WF.DefinitionSteps'; column does not allow nulls. UPDATE fails.
If I do a select instead of the update statement, like this:
SELECT source.DefinitionId, target.DefinitionId
FROM [CCHMergeIntermediate].[WF].[Definition] source
INNER JOIN [centralq3].[WF].[Definition] target
ON (((source.Name = target.Name) OR (source.Name IS NULL AND target.Name IS NULL)))
I get this result:
http://i.stack.imgur.com/3cZsM.png (sorry for external link, I don't have enaugh reputation to post image here )
What am I doing wrong? What I don't see? What am I missing..?

Answer 1

The problem was in the trigger at the condition. I modified the second where from i.oldPkCol = d.DefinitionId to i.oldPkCol = **d.oldPkCol** and it worked.
UPDATE WF.DefinitionSteps
SET DefinitionId =
(SELECT i.DefinitionId
FROM inserted i,
deleted d
WHERE WF.DefinitionSteps.DefinitionId = d.DefinitionId
AND i.oldPkCol = **d.oldPkCol**)
WHERE WF.DefinitionSteps.DefinitionId IN
(SELECT DefinitionId FROM deleted)