diff --git a/lib/tasks/merge_organisations.rake b/lib/tasks/merge_organisations.rake
index 32e0369fa..0df38a2ac 100644
--- a/lib/tasks/merge_organisations.rake
+++ b/lib/tasks/merge_organisations.rake
@@ -2,7 +2,7 @@ namespace :merge do
   desc "Merge organisations into one"
   task :merge_organisations, %i[absorbing_organisation_id merging_organisation_ids] => :environment do |_task, args|
     absorbing_organisation_id = args[:absorbing_organisation_id]
-    merging_organisation_ids = args[:merging_organisation_ids]
+    merging_organisation_ids = args[:merging_organisation_ids]&.split(",")&.map(&:to_i)
 
     raise "Usage: rake merge:merge_organisations[absorbing_organisation_id, merging_organisation_ids]" if merging_organisation_ids.blank? || absorbing_organisation_id.blank?
 
diff --git a/spec/lib/tasks/merge_organisations_spec.rb b/spec/lib/tasks/merge_organisations_spec.rb
index 2c102e73a..efb3fe99d 100644
--- a/spec/lib/tasks/merge_organisations_spec.rb
+++ b/spec/lib/tasks/merge_organisations_spec.rb
@@ -28,13 +28,13 @@ RSpec.describe "emails" do
       end
 
       it "raises an error when only merging organisations are given" do
-        expect { task.invoke(nil, [1, 2]) }.to raise_error(RuntimeError, "Usage: rake merge:merge_organisations[absorbing_organisation_id, merging_organisation_ids]")
+        expect { task.invoke(nil, "1,2") }.to raise_error(RuntimeError, "Usage: rake merge:merge_organisations[absorbing_organisation_id, merging_organisation_ids]")
       end
 
       it "raises runs the service with correct organisation IDs" do
         expect(Merge::MergeOrganisationsService).to receive(:new).with(absorbing_organisation_id: 1, merging_organisation_ids: [2, 3]).once
         expect(merge_organisations_service).to receive(:call).once
-        task.invoke(1, [2, 3])
+        task.invoke(1, "2,3")
       end
     end
   end