Error 137: Must Declare Variable ...

Hello,

I have the following SP, which gives out a "Error 137: Must declare variable @tmp_return_tbl" error.

This is the important part of the code:
.
.
.
-- DECLARE TABLE VARIABLE
DECLARE @tmp_return_tbl TABLE (tID int, Text_Title nvarchar(30), Text_Body nvarchar(100))

-- fill out table variable USING A SELECT FROM ANOTHER TABLE VARIABLE
-- NO PROBLEM HERE
INSERT INTO @tmp_return_tbl
SELECT TOP 1 * FROM @tmp_tbl
ORDER BY NEWID()

-- TRYING TO UPDATE A TABLE
UPDATE xTable
SET xTable.fieldY = xTable.fieldY + 1
WHERE xTable.tID = @tmp_return_tbl.tID --THIS PRODUCES THE ERROR
.
.
.

I know I cannot use a table variable in a JOIN without using an Alias, or use it directly in dynamic SQL (different scope) - but is this the problem here? What am I doing wrong?

Your help is much appreciated.