diff --git a/tests/test_checks_universal.py b/tests/test_checks_universal.py
index a66e5d7348..0e9a222173 100644
--- a/tests/test_checks_universal.py
+++ b/tests/test_checks_universal.py
@@ -1053,15 +1053,20 @@ def test_check_unreachable_glyphs():
         assert glyph not in message
 
     ttFont = TTFont(TEST_FILE("notosansmath/NotoSansMath-Regular.ttf"))
+    ttFont.ensureDecompiled()  # (required for mock glyph removal below)
+    glyph_order = ttFont.getGlyphOrder()
+
     # upWhiteMediumTriangle is used as a component in circledTriangle,
     # since CFF does not have composites it became unused.
     # So that is a build tooling issue.
     message = assert_results_contain(check(ttFont), WARN, "unreachable-glyphs")
     assert "upWhiteMediumTriangle" in message
-    assert "upWhiteMediumTriangle" in ttFont.glyphOrder
+    assert "upWhiteMediumTriangle" in glyph_order
 
-    # Other than that problem, no other glyphs are unreachable:
-    ttFont.glyphOrder.remove("upWhiteMediumTriangle")
+    # Other than that problem, no other glyphs are unreachable;
+    # Remove the glyph and then try again.
+    glyph_order.remove("upWhiteMediumTriangle")
+    ttFont.setGlyphOrder(glyph_order)
     assert "upWhiteMediumTriangle" not in ttFont.glyphOrder
     assert_PASS(check(ttFont))