SQL Error on Unnecessary varchar to int Conversion

Scenario

Consider the following table:

CREATE TABLE [dbo].[AuditTrail](

[AuditTrailID] [int] IDENTITY(1,1) NOT NULL,

[UserID] [int] NOT NULL,

[FieldName] [varchar](50) NULL,

[OldValue] [varchar](max) NULL,

[NewValue] [varchar](max) NULL

) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

And consider the following script:

DECLARE @OldRoleID int

DECLARE @NewRoleID int

DECLARE @OldUsername varchar(25)

DECLARE @NewUsername varchar(25)

SET @OldRoleID = 1

SET @NewRoleID = 2

SET @OldUsername = 'Juan'

SET @NewUsername = 'John'

INSERT INTO dbo.AuditTrail VALUES

(1, 'RoleID', @OldRoleID, @NewRoleID),

(1, 'Username', @OldUsername, @NewUsername);