Invalid operator for data type.

Kudos to y'all experts out there. I kinda needed your help. I'm trying to run a query...

SELECT a.AUF_POS AS Pos, c.ZL_STR AS Panel, a.POS_TEXT AS Description, a.BREITE AS W1, a.HOEHE

AS H1, a.BREITE2 AS W2, a.HOEHE2 AS H2, SUM(b.ANZ) AS Qty, SUM(b.LIEFER_ANZ) AS Dlvd,

SUM(b.RG_ANZ) AS Inv, (a.BREITE*a.HOEHE/CAST(1000000 AS NUMERIC)) AS UnitSQM,

(a.BREITE*a.HOEHE*SUM(b.ANZ)/CAST(1000000 AS NUMERIC)) as TotPosSQM

FROM liorder..LIORDER.AUF_POS a INNER JOIN liorder..LIORDER.AUF_STAT b ON a.AUF_NR = b.AUF_NR

AND a.AUF_POS = b.AUF_POS INNER JOIN liorder..LIORDER.AUF_TEXT c ON a.AUF_NR = c.AUF_NR AND

b.AUF_POS = c.AUF_POS

WHERE (c.ZL_MOD = 0) AND (b.AUF_NR = '86260')

GROUP BY a.AUF_POS, a.POS_TEXT, a.BREITE, a.BREITE2, a.HOEHE, a.HOEHE2, a.SFORM_NR, c.ZL_STR

...and I keep getting this error: Invalid operator for data type. Operator equals multiply, type equals nvarchar. I've tried every possible CAST and CONVERT but I just can't make it work. I'm pretty sure that the data types for the columns I mentioned on the mathematical equation are all numeric. Please help...Its going to be difficult for us to help without the DDL. Your query does alot of aggregrations (i.e. sum/avg/etc), thus you'd want to ensure that the column datatype is numeric.