Space In LIKE Parameter

I have a table with a Text column. A sproc that selects from this table has the following code:
ALTER PROCEDURE [dbo].[ErrorLogSelect]
(
@RecId INT = NULL,
@DateFrom DATETIME = NULL,
@DateTo DATETIME = NULL,
@SeverityFrom INT = NULL,
@SeverityTo INT = NULL,
@AppName VARCHAR(50) = NULL,
@ComputerName VARCHAR(32) = NULL,
@StackTrace VARCHAR(8000) = NULL,
)
AS

BEGIN
SELECT @StackTrace = '%' + @StackTrace + '%'

SET NOCOUNT ON;
SELECT RecID,
ErrDateTime,
ErrNumber,
ErrDescription,
StackTrace,
Source,
ProcName,
UserName,
ComputerName,
NetworkName,
AppVersion,
AppName,
AppExe,
OtherInfo,
Severity,
PCs.Location
FROM SWErrorLog LEFT OUTER JOIN
PCs ON ComputerName = dbo.PCs.PcNumber
WHERE RecId = COALESCE(@RecId, RecId) AND
ErrDateTime >= COALESCE(@DateFrom, ErrDateTime) AND
ErrDateTime <= COALESCE(@DateTo,ErrDateTime) AND
Severity >= COALESCE(@SeverityFrom, Severity) AND
Severity <= COALESCE(@SeverityTo, Severity) AND
AppName = COALESCE(@AppName, AppName) AND
ComputerName = COALESCE(@ComputerName, ComputerName)
AND StackTrace LIKE @StackTrace)
ORDER BY ErrDateTime DESC
END

When I execute this like this:
EXEC @return_value = [dbo].[ErrorLogSelect]
@RecId = NULL,
@DateFrom = NULL,
@DateTo = NULL,
@SeverityFrom = NULL,
@SeverityTo = NULL,
@AppName = NULL,
@ComputerName = NULL,
@StackTrace = 'OneWord',
@ErrDescription = NULL

it works just fine. However, this doesn't work:
EXEC @return_value = [dbo].[ErrorLogSelect]
@RecId = NULL,
@DateFrom = NULL,
@DateTo = NULL,
@SeverityFrom = NULL,
@SeverityTo = NULL,
@AppName = NULL,
@ComputerName = NULL,
@StackTrace = 'Two words',
@ErrDescription = NULL

The only difference is a space between the two words. Why should LIKE care about spaces between words? Does this have anything to do with the fact that I'm using a VARCHAR to filter a TEXT column?