PostGIS: Geometry column registering as GeometryCollection, not converting to MULTIPOLYGON

I'm trying to aggregate data from a tabular data source and then join it to a geometry file in PostGIS using a primary key.

CREATE TABLE example_geom
AS SELECT sl_adm2_sums_alias.* , adm2_code as district_pcode from gis_schema.sl_adm2
JOIN (select st_collect(sl_adm2.geom) AS geometry, sum(sl_sos.HEALTH) AS health_sum, sum(sl_sos.PROTECTION) AS protection_sum, sum(sl_sos.WASH) AS wash_sum, sum(sl_sos.cccm) AS cccm_sum, sum(sl_sos.food) AS food_sum, sum(sl_sos.livelihood) AS livelihood_sum, sum(sl_sos.education) AS education_sum, sum(sl_sos.agriculture) AS agriculture_sum, sum(sl_sos.HUMANITARIAN_ACCESS) AS humaccess_sum, sum(sl_sos.logistics) AS logistics_sum, sum(sl_sos.SHELTER_AND_NFI) AS shelter_and_wifi_sum, sl_sos.district
from sl_sos join gis_schema.sl_adm2 on sl_sos.DISTRICT_PCODE = sl_adm2.adm2_code group by sl_sos.district)
sl_adm2_sums_alias ON sl_adm2.adm2_name=sl_adm2_sums_alias.district;

and then

ALTER TABLE example_geom
ADD PRIMARY KEY (district_pcode);

The aggregation and join work fine but the geometry column in the resulting table does not appear to work. Further investigation reveals that the geometry column is registering as a GeometryCollection. I tried to change this with the following code.

ALTER TABLE example_geom ALTER COLUMN geometry
SET DATA TYPE geometry(MultiPolygon) USING ST_Multi(geometry);

This gave me the below error

ERROR: Geometry type (GeometryCollection) does not match column type (MultiPolygon)
SQL state: 22023

I've tried to fix this or get around it with no success. Could anyone help?

I'm using Postgres 9.4.1 with PostGIS version 2.1.5 on a Mac OS X 10.10.2.