diff --git a/amaranth/back/rtlil.py b/amaranth/back/rtlil.py
index fad0160..938bd32 100644
--- a/amaranth/back/rtlil.py
+++ b/amaranth/back/rtlil.py
@@ -664,7 +664,7 @@ class _LHSValueCompiler(_ValueCompiler):
         return wire_next or wire_curr
 
     def _prepare_value_for_Slice(self, value):
-        assert isinstance(value, (ast.Signal, ast.Slice, ast.Cat))
+        assert isinstance(value, (ast.Signal, ast.Slice, ast.Cat, ast.Part))
         return self(value)
 
     def on_Part(self, value):