stored procedure error: Conversion failed when converting the nvarchar value

I have the following SP:
set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [elevenboutique].[CategorySelectByClassID]
(
      @sortExpression nvarchar(50),
      @startRowIndex INT,
      @maximumRows INT,
      @ClassID INT
)
AS
IF LEN(@sortExpression) = 0
SET @sortExpression = 'CategoryName'

-- Since @startRowIndex is zero-based in the data Web control, but one-based w/ROW_NUMBER(), increment
SET @startRowIndex = @startRowIndex + 1
DECLARE @sql nvarchar(4000)

SET @sql ='SELECT  *
       FROM
(SELECT        *,
ROW_NUMBER() OVER(ORDER BY ' + @sortExpression + ') as RowNum
           FROM Category
WHERE        (ClassID = ' + CAST(@ClassID AS INT) + '))
AS Category
WHERE RowNum BETWEEN ' + CONVERT(nvarchar(10), @startRowIndex) +
       ' AND (' + CONVERT(nvarchar(10), @startRowIndex) + ' + '
                              + CONVERT(nvarchar(10), @maximumRows) + ') - 1'

-- Execute the SQL query
EXEC sp_executesql @sql

When I try to execute it with the following: EXEC CategorySelectByClassID 'CategoryName',0,100,305

I get the following error:
Msg 245, Level 16, State 1, Procedure CategorySelectByClassID, Line 16
Conversion failed when converting the nvarchar value 'SELECT  *
       FROM
(SELECT        *,
ROW_NUMBER() OVER(ORDER BY CategoryName) as RowNum
           FROM Category
WHERE        (ClassID = ' to data type int.

If I simply try to run this code I get no errors:
SELECT  *
       FROM
(SELECT        *,
ROW_NUMBER() OVER(ORDER BY CategoryName) as RowNum
           FROM Category
WHERE        (ClassID = 305))
AS Category