sql server varchar to int conversion error

i'm using sql server 2008

I have this query

DECLARE @EmployeeLastName AS VARCHAR(50)

-- Test Case 1 - Multiple Employees
--SET @EmployeeLastName = 'Johnson,Jackson,Roberts'

-- Test Case 2 - All Employees
--SET @EmployeeLastName = -1

-- Test Case 3 - Single Employee
SET @EmployeeLastName = 'Jackson'

-------------------

IF @EmployeeLastName LIKE '%,%'

	BEGIN  

	-- Test Case 1
	SELECT 'Multiple Employees Selected' AS EmployeesSelectedText 

	END  

ELSE IF @EmployeeLastName = -1

	BEGIN  

	-- Test Case 2
	SELECT 'All Employees' AS EmployeesSelectedText 

	END

--ELSE
ELSE 

	BEGIN  

	SELECT 'Single Employee Selected' AS EmployeesSelectedText 

	END;

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When I use Test Case 1 Parameter  like this it works fine:


When I use Test Case 2 Parameter  like this it works fine:


When I use Test Case 3 Parameter  like this I get an error



Any idea of why I'm getting an error when I use this ?

SET @EmployeeLastName = 'Jackson'