concatenate question

Here's my table:

plsql_dw=# \d tmpsv_parent_master          Table "staging.tmpsv_parent_master"    Column     |            Type
  | Modifiers 
 
----------------+-----------------------------+-----------memberid       | numeric                     | addeddate
|timestamp without time zone | sourceid       | numeric                     | regcomplete    | numeric(1,0)
  | optoutdate     | date                        | bouncedate     | date                        | websiteid      |
numeric                    | emailbounced   | numeric(2,0)                | emailok        | numeric(2,0)
| emailaddress   | character varying(50)       | srcwebsiteid   | numeric                     | srcmemberid    |
numeric                    | sitetype       | character varying           | commissionpct  | numeric
| pricepermember | numeric                     | acceptrate     | numeric(3,2)                | mktgcenterid   |
numeric                    | label          | character varying(32)       | 
 


Why won't this work?
plsql_dw=# select memberid || addeddate from tmpsv_parent_master limit
10;
ERROR:  operator does not exist: numeric || timestamp without time zone
LINE 1: select memberid || addeddate from tmpsv_parent_master limit ...                       ^
HINT:  No operator matches the given name and argument type(s). You
might need to add explicit type casts.

Thanks.