expected both swapped tables to have TOAST tables

I tried to truncate a table and received the following error:
"expected both swapped tables to have TOAST tables".

Googling provided no insight. What does it mean?

Version 7.4.1.

Note: I can delete from the table and vacuum full to empty it, I just
can't truncate it. I don't see anything special with this table and
can truncate a nearly identical one. Here's the structure:

Table "foobar"
 Column |           Type           |                 Modifiers
--------+--------------------------+-------------------------------------------
 asof   | timestamp with time zone | default
date_trunc('second'::text, now())
 r0     | smallint                 |
 r1     | character varying(30)    |
 r2     | character varying(30)    |
 r3     | character varying(30)    |
 r4     | character varying(30)    |
 r5     | character varying(30)    |
 r6     | character varying(30)    |

Cheers,
Steve