diff --git a/Lib/fontTools/otlLib/builder.py b/Lib/fontTools/otlLib/builder.py
index 7e1444518..7711fa4c5 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 TypeError(f"Glyph {glyph} already present in class.")
             self.glyphs_[glyph] = glyphs
 
     def classes(self):
diff --git a/Tests/otlLib/builder_test.py b/Tests/otlLib/builder_test.py
index bdfc64509..a2c51331d 100644
--- a/Tests/otlLib/builder_test.py
+++ b/Tests/otlLib/builder_test.py
@@ -1101,6 +1101,18 @@ 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", "d"})
+        with pytest.raises(TypeError):
+            b.add({"a"})
+        with pytest.raises(TypeError):
+            b.add({"b"})
+        with pytest.raises(TypeError):
+            b.add({"c"})
+        with pytest.raises(TypeError):
+            b.add({"d"})
+
 
 buildStatTable_test_data = [
     ([