[GENERAL] 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