diff --git a/libyul/optimiser/KnowledgeBase.cpp b/libyul/optimiser/KnowledgeBase.cpp
index 00d7cda1c..36f07b74a 100644
--- a/libyul/optimiser/KnowledgeBase.cpp
+++ b/libyul/optimiser/KnowledgeBase.cpp
@@ -52,7 +52,7 @@ optional<u256> KnowledgeBase::differenceIfKnownConstant(YulString _a, YulString
 	if (offA.reference == offB.reference)
 		return offA.offset - offB.offset;
 	else
-		return {};
+		return nullopt;
 }