Common Table Expression Error

I have a requirement to rewrite some of our simple select statement queries using a CTE. And I have had no problems doing so until I encountered one from a query like the one below:

USE AdventureWorks2012
GO
DECLARE @TotalRows INT 
SELECT @TotalRows = COUNT(CustomerID)
FROM Sales.Customer
WHERE CustomerID IN (SELECT CustomerID FROM Sales.SalesOrderHeader);

SELECT @TotalRows AS 'T'

And I rewrote it like the one below but I am getting error.

DECLARE @TotalRows INT 
;WITH TotalRows
(
SELECT @TotalRows = COUNT(CustomerID)
FROM Sales.Customer
WHERE CustomerID IN (SELECT CustomerID FROM Sales.SalesOrderHeader))
SELECT * FROM TotalRows

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