diff --git a/amaranth/lib/data.py b/amaranth/lib/data.py
index 4bd010d..1252792 100644
--- a/amaranth/lib/data.py
+++ b/amaranth/lib/data.py
@@ -419,6 +419,11 @@ class _AggregateMeta(ShapeCastable, type):
                 if name in namespace:
                     reset[name] = namespace.pop(name)
             cls = type.__new__(metacls, name, bases, namespace)
+            if cls.__layout_cls is UnionLayout:
+                if len(reset) > 1:
+                    raise ValueError("Reset value for at most one field can be provided for "
+                                     "a union class (specified: {})"
+                                     .format(", ".join(reset.keys())))
             cls.__layout = cls.__layout_cls(layout)
             cls.__reset  = reset
             return cls
diff --git a/tests/test_lib_data.py b/tests/test_lib_data.py
index 6ed9693..6b9d8f7 100644
--- a/tests/test_lib_data.py
+++ b/tests/test_lib_data.py
@@ -679,6 +679,14 @@ class UnionTestCase(FHDLTestCase):
         self.assertEqual(s.attrs, {"debug": 1})
         self.assertEqual(s.decoder, decoder)
 
+    def test_define_reset_two_wrong(self):
+        with self.assertRaisesRegex(ValueError,
+                r"^Reset value for at most one field can be provided for a union class "
+                r"\(specified: a, b\)$"):
+            class U(Union):
+                a: unsigned(1) = 1
+                b: unsigned(2) = 1
+
     def test_construct_reset_two_wrong(self):
         class U(Union):
             a: unsigned(1)