diff --git a/kernel/rtlil.h b/kernel/rtlil.h
index 868aaaa14..1ed055715 100644
--- a/kernel/rtlil.h
+++ b/kernel/rtlil.h
@@ -734,8 +734,8 @@ public:
 	inline int size() const { return width_; }
 	inline bool empty() const { return width_ == 0; }
 
-	inline RTLIL::SigBit &operator[](int index) { inline_unpack(); return bits_.at(index); }
-	inline const RTLIL::SigBit &operator[](int index) const { inline_unpack(); return bits_.at(index); }
+	inline RTLIL::SigBit &operator[](int index) { inline_unpack(); return index >= 0 ? bits_.at(index) : bits_.at(width_ + index); }
+	inline const RTLIL::SigBit &operator[](int index) const { inline_unpack(); return index >= 0 ? bits_.at(index) : bits_.at(width_ + index); }
 
 	inline RTLIL::SigSpecIterator begin() { RTLIL::SigSpecIterator it; it.sig_p = this; it.index = 0; return it; }
 	inline RTLIL::SigSpecIterator end() { RTLIL::SigSpecIterator it; it.sig_p = this; it.index = width_; return it; }