Embedded SQL Select Into

Sigh...so I'm trying this:

Exec SQL
Select Session_Id Into :WkSession_ID
From MblSesP
Where Session_End is null;

I've defined Wk_Session_ID as:

D WkSession_ID S 128
D Dim(1000)

Note: I've also tried to define it as a data structure array (or thought I
did). I get an SQL0312 error - variable not found or unusable.

The definition of Session_ID (from DSPFFD) is:

SESSION_ID CHAR 128 130 43 Both SESSION_ID
Variable length field -- Allocated length : None
Default value . . . . . . . . . . . . . . : None
Coded Character Set Identifier . . . . . : 37

Two questions: What am I doing wrong, and why does SQL hate me?

Thanks!