diff --git a/newsfragments/5084.bugfix.rst b/newsfragments/5084.bugfix.rst
new file mode 100644
index 0000000000..f35ca12b0d
--- /dev/null
+++ b/newsfragments/5084.bugfix.rst
@@ -0,0 +1,2 @@
+Fixed a bug where calling ``build_ext.finalize_options`` after ``define`` or
+``undef`` attributes were already set would raise an exception.
diff --git a/setuptools/_distutils/command/build_ext.py b/setuptools/_distutils/command/build_ext.py
index ec45b4403e..6e61e56777 100644
--- a/setuptools/_distutils/command/build_ext.py
+++ b/setuptools/_distutils/command/build_ext.py
@@ -266,14 +266,14 @@ def finalize_options(self) -> None:  # noqa: C901
         # specified by the 'define' option will be set to '1'.  Multiple
         # symbols can be separated with commas.
 
-        if self.define:
+        if isinstance(self.define, str):
             defines = self.define.split(',')
             self.define = [(symbol, '1') for symbol in defines]
 
         # The option for macros to undefine is also a string from the
         # option parsing, but has to be a list.  Multiple symbols can also
         # be separated with commas here.
-        if self.undef:
+        if isinstance(self.undef, str):
             self.undef = self.undef.split(',')
 
         if self.swig_opts is None:
diff --git a/setuptools/tests/test_build_ext.py b/setuptools/tests/test_build_ext.py
index c7b60ac32f..d7525995c6 100644
--- a/setuptools/tests/test_build_ext.py
+++ b/setuptools/tests/test_build_ext.py
@@ -178,6 +178,22 @@ def C(file):
         assert example_stub.startswith(f"{build_lib}/mypkg/__pycache__/ext1")
         assert example_stub.endswith(".pyc")
 
+    def test_finalize_options_subclassing(self):
+        """
+        Regression test. Check that calling build_ext.finalize_options after
+        define or undef attributes were already set to their final type doesn't raise.
+        """
+        extension = Extension('spam.eggs', ['eggs.c'])
+        dist = Distribution(dict(ext_modules=[extension]))
+        cmd = build_ext(dist)
+        cmd.define = [("MY_MACRO", "1")]
+        cmd.undef = ["EVIL_MACRO"]
+
+        cmd.finalize_options()
+
+        assert cmd.define == [("MY_MACRO", "1")]
+        assert cmd.undef == ["EVIL_MACRO"]
+
 
 class TestBuildExtInplace:
     def get_build_ext_cmd(self, optional: bool, **opts) -> build_ext: