Tsql - updating an array from a select statement

I have two tables. TableA and TableB.

TableA holds a varbinary(max) column - named [BinaryA] TableB holds a column (named "Volume", type "Long") that contains each varbinary volume.

in order to select all volumes I query

SELECT ID, MyVolume = DATALENGTH([Binary]) FROM [VAULT].[TABLEA]

than, I want to update tableB with its volume.

I then write

update [TableB] set [VOLUME] = ( SELECT MyVolume = DATALENGTH([Binary]) FROM [VAULT].[TABLEA] ab WHERE id = ab.[Id])

I receive than

Cannot insert the value NULL into column 'Volume', table 'MySchema.Asset'; column does not allow nulls. UPDATE fails.

Though I dont receive any NULL when I run

SELECT ID, MyVolume = DATALENGTH([Binary]) FROM [VAULT].[TABLEA]

Try with this query:

UPDATE TableB SET TableB.[VOLUME] = DATALENGTH([T2.Binary]) FROM TableB INNER JOIN [VAULT].[TABLEA] T2 ON TableB.TAL_ID = T2.TAL_ID Assuming that TableB and [VAULT].[TABLEA] are related by ID field.