diff --git a/kernel/sigtools.h b/kernel/sigtools.h
index 69d6cd72d..7f7f95ab6 100644
--- a/kernel/sigtools.h
+++ b/kernel/sigtools.h
@@ -68,7 +68,7 @@ struct SigPool
 	void del(const SigPool &other)
 	{
 		for (auto &bit : other.bits)
-			bits.insert(bit);
+			bits.erase(bit);
 	}
 
 	void expand(RTLIL::SigSpec from, RTLIL::SigSpec to)