Error using parameters in a dynamic sql query

I'm having an issue using parameters in a dynamic sql stored procedure.  I receive "Must declare the scalar variable "@StartIdex"." when I execute the stored procedure.   The issue is being caused by the two integer parameters that are being referenced in dynamic sql.  Can someone help me resolve this issue ?

        @Empid nvarchar(50) = null,
	 @FirstName nvarchar(80) = null,
	 @LastName nvarchar(80) = null,
	 @StartIndex int,
	 @MaximumRows int
	 AS
BEGIN
SET NOCOUNT ON;
Declare @SQLQuery nvarchar(4000)
SET @Query = ' 
    Select RowNum,Empid,firstname,lastname From
	(SELECT ROW_NUMBER() OVER(ORDER BY empid) as RowNum,  
	  empid,
	  firstname,
	  lastname,
          WHERE firstname is not null
	) as employee 
	WHERE RowNum BETWEEN @StartIndex AND (@StartIndex + @MaximumRows) - 1'
	If @Empid is not null
		Set @Query = @Query + ' And (Empid = @Empid)'
	If @FirstName is not null
		Set @Query = @Query + ' And (FirstName like @FirstName + ''%'')'
	If @LastName is not null
		Set @Query = @Query + ' And (LastName like @LastName + ''%'')'
	Execute sp_Executesql @Query