Reading a Sum(field) column in RPGSQLLE

I am trying to total a an amount on a number of records in a file within an RPG program using embedded SQL.

I am building up the SQL statement using:

C* Build the SQL Statement for the current line... C Eval wSql = 'select sum(' + %Trim(CAENVN) + C ') into ? from ' + %Trim(CAEMVN) C* If selection criteria was passed and it isn't *NONE then add it to
C* the SQL statement... C If CAAGNC <> *Blanks and CAAGNC <> '*NONE'
C Eval wSql = %Trim(wSql) + ' where ' + CAAGNC
C EndIf
And then preparing and running it using:

C* Now execute the SQL statement...
C/Exec SQL C+ Prepare prpSQL from :wSQL C/End-Exec C/Exec SQL C+ Execute prpSQL using :wTotal C/End-Exec
However, I am getting an error in the joblog that says:

Token ? was not valid. Valid tokens: : <IDENTIFIER>.

Can anybody tell me what I'm doing wrong? I thought I had to use parameter markers for the variable name.

Thanks

Jonathan