diff --git a/music21/scale/__init__.py b/music21/scale/__init__.py
index 9dc898f0f..6e298ee90 100644
--- a/music21/scale/__init__.py
+++ b/music21/scale/__init__.py
@@ -1016,7 +1016,7 @@ def buildNetwork(self, mode: t.Any = None) -> None:
         intervalSum = interval.add(mode)
         iComplement = intervalSum.complement
         if iComplement is not None:
-            mode.append(iComplement)
+            mode = [*mode, iComplement]  # a copy: mode is the caller's list
 
         # steps wider than an octave complete a pattern spanning several
         # octaves, which does not repeat every octave
diff --git a/music21/scale/test_scale_main.py b/music21/scale/test_scale_main.py
index 96876fc2c..e8aac4c8a 100644
--- a/music21/scale/test_scale_main.py
+++ b/music21/scale/test_scale_main.py
@@ -233,6 +233,15 @@ def testBasic(self):
              <music21.pitch.Pitch F#5>],
  'notMatched': [<music21.pitch.Pitch C#5>, <music21.pitch.Pitch G#5>]}''', pformat(sc1.match(sc2)))
 
+    def testOctaveRepeatingScaleLeavesModeAlone(self):
+        mode = ['M3', 'M3']
+        sc = scale.OctaveRepeatingScale('c4', mode)
+        self.assertEqual(mode, ['M3', 'M3'])
+        self.assertEqual(self.pitchOut(sc.pitches), '[C4, E4, G#4, C5]')
+
+        again = scale.OctaveRepeatingScale('c4', mode)
+        self.assertEqual(self.pitchOut(again.pitches), '[C4, E4, G#4, C5]')
+
     def testOctaveRepeatingScaleWiderThanAnOctave(self):
         sc = scale.OctaveRepeatingScale('c4', ['P5', 'P5'])
         self.assertEqual(self.pitchOut(sc.pitches), '[C4, G4, D5, C6]')