PG 10 - Trouble with overlap of range partition of two dimensions

So I have a table that has two fields I want to partition by:

CREATE TABLE lotsa_data (
start_time timestamp with time zone,
source_no integer,
counter integer)
PARTITION BY RANGE (start_time, source_no);

CREATE TABLE lotsa_data_20171027_src1 PARTITION OF lotsa_data
FOR VALUES FROM ('2017-10-26 18:00:00-06', 1) TO ('2017-10-27 17:59:59.999-06', 1);
(Works fine)

CREATE TABLE lotsa_data_20171027_src3 PARTITION OF lotsa_data
FOR VALUES FROM ('2017-10-26 18:00:00-06', 3) TO ('2017-10-27 17:59:59.999-06', 3);

ERROR: partition " lotsa_data_20171027_src1" would overlap partition "lotsa_data_20171027_src3"

Why am I getting this error? (Also, if I go "FROM ('2017-10-26 00:00:00 UTC') TO ('2017-10-27 00:00:00 UTC')" I also get overlap errors.

Thanks for your help ...
--Stephen

Stephen Froehlich
Sr. Strategist, CableLabs(r)

s(dot)froehlich(at)cablelabs(dot)com
Tel: +1 (303) 661-3708