"duplicate merge key detected in join" error on tables without keys

I’m trying to use MERGE to upsert aggregate data into a collection of tables.  These tables each have a set of fields and a set of metrics.  For each table some fields are nullable and some are not.  No keys are declared on the tables.  I’m using temp tables, which match the target tables, to hold the aggregate data, which is then consumed by the MERGE statement.  When I run the MERGE statements I receive a “Duplicate MERGE key detected in join” error, which shows that it is treating only the not null columns as a key despite my ON clause including syntax to include the null columns in the match.  

Sample pseudo syntax:
CREATE TABLE target (
fieldA varchar not null,
fieldB varchar not null,
fieldC varchar null,
metricA int,
metricB int);

CREATE TABLE temp (
fieldA varchar not null,
fieldB varchar not null,
fieldC varchar null,
metricA int,
metricB int);

MERGE INTO target trg
USING temp tmp ON (
    trg.FieldA = tmp.FieldA
    AND trg.FieldB = tmp.FieldB
    AND ((trg.FieldC IS NULL AND tmp.FieldC IS NULL) OR trg.FieldC = tmp.FieldC))
WHEN MATCHED THE UPDATE
    SET trg.MetricA = tmp.MetricA ,
    trg.MetricB = tmp.MetricB
WHEN NOT MATCHED THEN INSERT (
    FieldA ,
    FieldB ,
    FieldC ,
    MetricA ,
    MetricB )
VALUES (
    tmp.FieldA ,
    tmp.FieldB ,
    tmp.FieldC ,
    tmp.MetricA ,
    tmp.MetricB );