diff --git a/nmigen/hdl/ast.py b/nmigen/hdl/ast.py
index 9295ed4..56c61ca 100644
--- a/nmigen/hdl/ast.py
+++ b/nmigen/hdl/ast.py
@@ -361,7 +361,7 @@ class Slice(Value):
             raise TypeError("Slice end must be an integer, not '{!r}'".format(end))
 
         n = len(value)
-        if start not in range(-n, n):
+        if start not in range(-(n+1), n+1):
             raise IndexError("Cannot start slice {} bits into {}-bit value".format(start, n))
         if start < 0:
             start += n