diff --git a/Lib/fontTools/otlLib/builder.py b/Lib/fontTools/otlLib/builder.py
index 7e1444518..029aa3fc5 100644
--- a/Lib/fontTools/otlLib/builder.py
+++ b/Lib/fontTools/otlLib/builder.py
@@ -2574,7 +2574,8 @@ class ClassDefBuilder(object):
             return
         self.classes_.add(glyphs)
         for glyph in glyphs:
-            assert glyph not in self.glyphs_
+            if glyph in self.glyphs_:
+                raise OpenTypeLibError(f"Glyph {glyph} is already present in class.", None)
             self.glyphs_[glyph] = glyphs
 
     def classes(self):
diff --git a/Tests/otlLib/builder_test.py b/Tests/otlLib/builder_test.py
index bdfc64509..3ea5a7459 100644
--- a/Tests/otlLib/builder_test.py
+++ b/Tests/otlLib/builder_test.py
@@ -2,7 +2,7 @@ import io
 import struct
 from fontTools.misc.fixedTools import floatToFixed
 from fontTools.misc.testTools import getXML
-from fontTools.otlLib import builder
+from fontTools.otlLib import builder, error
 from fontTools import ttLib
 from fontTools.ttLib.tables import otTables
 import pytest
@@ -1101,6 +1101,12 @@ class ClassDefBuilderTest(object):
         assert not b.canAdd({"d", "e", "f"})
         assert not b.canAdd({"f"})
 
+    def test_add_exception(self):
+        b = builder.ClassDefBuilder(useClass0=True)
+        b.add({"a", "b", "c"})
+        with pytest.raises(error.OpenTypeLibError):
+            b.add({"a", "d"})
+
 
 buildStatTable_test_data = [
     ([