diff --git a/libyul/optimiser/SimplificationRules.cpp b/libyul/optimiser/SimplificationRules.cpp
index b3190fefb..8ed63fa8d 100644
--- a/libyul/optimiser/SimplificationRules.cpp
+++ b/libyul/optimiser/SimplificationRules.cpp
@@ -114,7 +114,8 @@ bool Pattern::matches(Expression const& _expr, map<YulString, Expression const*>
 	{
 		YulString varName = boost::get<Identifier>(_expr).name;
 		if (_ssaValues.count(varName))
-			expr = _ssaValues.at(varName);
+			if (Expression const* new_expr = _ssaValues.at(varName))
+				expr = new_expr;
 	}
 	assertThrow(expr, OptimizerException, "");
 
diff --git a/test/libyul/yulOptimizerTests/expressionSimplifier/unassigned_vars.yul b/test/libyul/yulOptimizerTests/expressionSimplifier/unassigned_vars.yul
new file mode 100644
index 000000000..87391b8c8
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/expressionSimplifier/unassigned_vars.yul
@@ -0,0 +1,14 @@
+// c & d can't be optimized as expression simplifier doesn't handle default
+// values yet
+{
+	let c
+	let d
+	let y := add(d, add(c, 7))
+}
+// ----
+// expressionSimplifier
+// {
+//     let c
+//     let d
+//     let y := add(add(d, c), 7)
+// }