Erro do SQL Server: dados de seqüência de caracteres ou binários seriam truncados

Minha mesa :

log_id                 bigint
old_value                  xml
new_value                  xml
module                 varchar(50)
reference_id           bigint
[transaction]          varchar(100)
transaction_status         varchar(10)
stack_trace                ntext
modified_on                datetime
modified_by                bigint

Inserir consulta:

INSERT INTO [dbo].[audit_log]
           ([old_value],[new_value],[module],[reference_id],[transaction]
           ,[transaction_status],[stack_trace],[modified_on],[modified_by])
     VALUES
            ('asdf','asdf','Subscriber',4,'_transaction',
            '_transaction_status','_stack_trace',getdate(),555)

Erro:

Msg 8152, Level 16, State 14, Line 1
String or binary data would be truncated.
The statement has been terminated.

Por que é que ???

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