diff --git a/CHANGELOG.md b/CHANGELOG.md
index d012dcb..671fba4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 # Changelog
 
 ## Unreleased
+* Fix sample building for TS.MADD with multiple series (#77)
 
 ## 0.7.1
 * Handle ActiveSupport::TimeWithZone objects (#75)
diff --git a/lib/redis/time_series.rb b/lib/redis/time_series.rb
index 61105ad..8a2e241 100644
--- a/lib/redis/time_series.rb
+++ b/lib/redis/time_series.rb
@@ -108,7 +108,7 @@ def destroy(key)
       #
       def madd(data)
         data.reduce([]) do |memo, (key, value)|
-          memo << parse_madd_values(key, value)
+          memo += parse_madd_values(key, value)
           memo
         end.then do |args|
           cmd('TS.MADD', args).each_with_index.map do |result, idx|
diff --git a/spec/redis/time_series_spec.rb b/spec/redis/time_series_spec.rb
index 7b0bd65..e0731fa 100644
--- a/spec/redis/time_series_spec.rb
+++ b/spec/redis/time_series_spec.rb
@@ -188,6 +188,14 @@ def msec(ts)
         end.to issue_command "TS.MADD foo #{ts_msec} 1 foo #{ts_msec + 1} 2 foo #{ts_msec + 2} 3 "\
         "bar #{ts_msec} 4 bar #{ts_msec + 1} 5 bar #{ts_msec + 2} 6 bar #{ts_msec + 3} 7"
       end
+
+      it 'correctly returns samples' do
+        described_class.create(:foo)
+        expect(described_class.madd(foo: { 123 => 1, 456 => 2 })).to all(
+          be_a(Redis::TimeSeries::Sample)
+        )
+        described_class.destroy(:foo)
+      end
     end
   end