A call to upsert (Table 1 0) [TableCount +=. 1] generates an SQL query like

INSERT INTO "table" ("unique", "count") VALUES (1,0)
ON CONFLICT ("unique") DO UPDATE
SET "count"="count"+1
WHERE "table"."unique"=1
RETURNING "table"."id", "table"."unique", "table"."count"

Now PostgreSQL complains:

ERROR:  column reference "count" is ambiguous
LINE 3: SET "count"="count"+1

To fix this, column "count" should be qualified with the table. Using SET "count"="table"."count"+1 works as expected.

I tested this with PostgreSQL 9.6.2 and Persistent 2.6. Older versions of persistent where multiple queries were used did not have the problem.