diff --git a/amaranth/lib/enum.py b/amaranth/lib/enum.py
index 9748b28..c1e6f03 100644
--- a/amaranth/lib/enum.py
+++ b/amaranth/lib/enum.py
@@ -30,34 +30,42 @@ class EnumMeta(ShapeCastable, py_enum.EnumMeta):
         return super().__prepare__(name, bases, **kwargs)
 
     def __new__(metacls, name, bases, namespace, shape=None, **kwargs):
-        cls = py_enum.EnumMeta.__new__(metacls, name, bases, namespace, **kwargs)
         if shape is not None:
-            # Shape is provided explicitly. Set the `_amaranth_shape_` attribute, and check that
-            # the values of every member can be cast to the provided shape without truncation.
-            cls._amaranth_shape_ = shape = Shape.cast(shape)
-            for member in cls:
-                try:
-                    member_shape = Const.cast(member.value).shape()
-                except TypeError as e:
-                    raise TypeError("Value of enumeration member {!r} must be "
+            shape = Shape.cast(shape)
+        for member_name, member_value in namespace.items():
+            if py_enum._is_sunder(member_name) or py_enum._is_dunder(member_name):
+                continue
+            try:
+                member_shape = Const.cast(member_value).shape()
+            except TypeError as e:
+                if shape is not None:
+                    raise TypeError("Value {!r} of enumeration member {!r} must be "
                                     "a constant-castable expression"
-                                    .format(member)) from e
+                                    .format(member_value, member_name)) from e
+                else:
+                    continue
+            if shape is not None:
                 if member_shape.signed and not shape.signed:
                     warnings.warn(
-                        message="Value of enumeration member {!r} is signed, but the enumeration "
-                                "shape is {!r}" # the repr will be `unsigned(X)`
-                                .format(member, shape),
+                        message="Value {!r} of enumeration member {!r} is signed, but "
+                                "the enumeration shape is {!r}" # the repr will be `unsigned(X)`
+                                .format(member_value, member_name, shape),
                         category=SyntaxWarning,
                         stacklevel=2)
                 elif (member_shape.width > shape.width or
                       member_shape.width == shape.width and
                         shape.signed and not member_shape.signed):
                     warnings.warn(
-                        message="Value of enumeration member {!r} will be truncated to "
+                        message="Value {!r} of enumeration member {!r} will be truncated to "
                                 "the enumeration shape {!r}"
-                                .format(member, shape),
+                                .format(member_value, member_name, shape),
                         category=SyntaxWarning,
                         stacklevel=2)
+        cls = py_enum.EnumMeta.__new__(metacls, name, bases, namespace, **kwargs)
+        if shape is not None:
+            # Shape is provided explicitly. Set the `_amaranth_shape_` attribute, and check that
+            # the values of every member can be cast to the provided shape without truncation.
+            cls._amaranth_shape_ = shape
         else:
             # Shape is not provided explicitly. Behave the same as a standard enumeration;
             # the lack of `_amaranth_shape_` attribute is used to emit a warning when such
diff --git a/tests/test_lib_enum.py b/tests/test_lib_enum.py
index 60bdc5f..49d0a2b 100644
--- a/tests/test_lib_enum.py
+++ b/tests/test_lib_enum.py
@@ -12,8 +12,7 @@ class EnumTestCase(FHDLTestCase):
 
     def test_non_int_members_wrong(self):
         with self.assertRaisesRegex(TypeError,
-                r"^Value of enumeration member <EnumA\.A: 'str'> must be "
-                r"a constant-castable expression$"):
+                r"^Value 'str' of enumeration member 'A' must be a constant-castable expression$"):
             class EnumA(Enum, shape=unsigned(1)):
                 A = "str"
 
@@ -54,24 +53,24 @@ class EnumTestCase(FHDLTestCase):
 
     def test_shape_explicit_wrong_signed_mismatch(self):
         with self.assertWarnsRegex(SyntaxWarning,
-                r"^Value of enumeration member <EnumA\.A: -1> is signed, but the enumeration "
+                r"^Value -1 of enumeration member 'A' is signed, but the enumeration "
                 r"shape is unsigned\(1\)$"):
             class EnumA(Enum, shape=unsigned(1)):
                 A = -1
 
     def test_shape_explicit_wrong_too_wide(self):
         with self.assertWarnsRegex(SyntaxWarning,
-                r"^Value of enumeration member <EnumA\.A: 2> will be truncated to the enumeration "
+                r"^Value 2 of enumeration member 'A' will be truncated to the enumeration "
                 r"shape unsigned\(1\)$"):
             class EnumA(Enum, shape=unsigned(1)):
                 A = 2
         with self.assertWarnsRegex(SyntaxWarning,
-                r"^Value of enumeration member <EnumB\.A: 1> will be truncated to the enumeration "
+                r"^Value 1 of enumeration member 'A' will be truncated to the enumeration "
                 r"shape signed\(1\)$"):
             class EnumB(Enum, shape=signed(1)):
                 A = 1
         with self.assertWarnsRegex(SyntaxWarning,
-                r"^Value of enumeration member <EnumC\.A: -2> will be truncated to the "
+                r"^Value -2 of enumeration member 'A' will be truncated to the "
                 r"enumeration shape signed\(1\)$"):
             class EnumC(Enum, shape=signed(1)):
                 A = -2