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.