Merge Syntax with NULL Values

Question

Say I want to compare my Source and Target Table using MERGE syntax in SQL

I want to update the End_Dt of my target table when target.close_Date is not equal to Source.Close_Date.

Table 1

ID   Close_Date   End_Date
1    NULL         9999-12-31 

Table 2

ID   Close_Date
1    2019-01-31 

Merge Table1 as Target
Using Table 2 as Source
On (Target.ID = Source.ID_)
WHEN MATCHED and 
isnull(Target.Closed_Date,'9999-12-31') = isnull(Source.Closed_Date,'9999-12-31')
THEN
Update Set Table1.End_Dt = cast(getdate() as date));

After I ran the code nothing is updated in my target table.