diff --git a/amaranth/hdl/ast.py b/amaranth/hdl/ast.py
index e1ee694..3fdb031 100644
--- a/amaranth/hdl/ast.py
+++ b/amaranth/hdl/ast.py
@@ -860,9 +860,14 @@ class Part(Value):
         if not isinstance(stride, int) or stride <= 0:
             raise TypeError("Part stride must be a positive integer, not {!r}".format(stride))
 
+        value = Value.cast(value)
+        offset = Value.cast(offset)
+        if offset.shape().signed:
+            raise TypeError("Part offset must be unsigned")
+
         super().__init__(src_loc_at=src_loc_at)
-        self.value  = Value.cast(value)
-        self.offset = Value.cast(offset)
+        self.value  = value
+        self.offset = offset
         self.width  = width
         self.stride = stride
 
diff --git a/tests/test_hdl_ast.py b/tests/test_hdl_ast.py
index 0cb8865..760744c 100644
--- a/tests/test_hdl_ast.py
+++ b/tests/test_hdl_ast.py
@@ -785,6 +785,11 @@ class BitSelectTestCase(FHDLTestCase):
         s = self.c.bit_select(self.s, 2)
         self.assertEqual(repr(s), "(part (const 8'd0) (sig s) 2 1)")
 
+    def test_offset_wrong(self):
+        with self.assertRaisesRegex(TypeError,
+                r"^Part offset must be unsigned$"):
+            self.c.bit_select(self.s.as_signed(), 1)
+
 
 class WordSelectTestCase(FHDLTestCase):
     def setUp(self):
@@ -817,6 +822,11 @@ class WordSelectTestCase(FHDLTestCase):
         s = self.c.word_select(self.s, 2)
         self.assertEqual(repr(s), "(part (const 8'd0) (sig s) 2 2)")
 
+    def test_offset_wrong(self):
+        with self.assertRaisesRegex(TypeError,
+                r"^Part offset must be unsigned$"):
+            self.c.word_select(self.s.as_signed(), 1)
+
 
 class CatTestCase(FHDLTestCase):
     def test_shape(self):