diff --git a/amaranth/lib/data.py b/amaranth/lib/data.py
index 237cc93..605b1fc 100644
--- a/amaranth/lib/data.py
+++ b/amaranth/lib/data.py
@@ -240,8 +240,9 @@ class Layout(ShapeCastable, metaclass=ABCMeta):
                                              key_value.shape()))
             elif not isinstance(key_value, Const):
                 key_value = Const(key_value, cast_field_shape)
-            int_value &= ~(((1 << cast_field_shape.width) - 1) << field.offset)
-            int_value |= key_value.value << field.offset
+            mask = ((1 << cast_field_shape.width) - 1) << field.offset
+            int_value &= ~mask
+            int_value |= (key_value.value << field.offset) & mask
         return Const(int_value, self.as_shape())
 
 
diff --git a/tests/test_lib_data.py b/tests/test_lib_data.py
index 845d240..2dd1928 100644
--- a/tests/test_lib_data.py
+++ b/tests/test_lib_data.py
@@ -398,6 +398,12 @@ class LayoutTestCase(FHDLTestCase):
         self.assertRepr(fl.const({"b": 0b10}), "(const 2'd2)")
         self.assertRepr(fl.const({"a": 0b1, "b": 0b10}), "(const 2'd2)")
 
+        sls = StructLayout({
+            "a": signed(4),
+            "b": signed(4)
+        })
+        self.assertRepr(sls.const({"b": 0, "a": -1}), "(const 8'd15)")
+
     def test_const_wrong(self):
         sl = StructLayout({"f": unsigned(1)})
         with self.assertRaisesRegex(TypeError,