diff --git a/src/storm/storage/expressions/ToRationalNumberVisitor.cpp b/src/storm/storage/expressions/ToRationalNumberVisitor.cpp
index 26ad15b5a..c8931c7f3 100644
--- a/src/storm/storage/expressions/ToRationalNumberVisitor.cpp
+++ b/src/storm/storage/expressions/ToRationalNumberVisitor.cpp
@@ -7,6 +7,11 @@
 
 namespace storm {
     namespace expressions {
+    template<typename RationalNumberType>
+        ToRationalNumberVisitor<RationalNumberType>::ToRationalNumberVisitor() : ExpressionVisitor() {
+            // Intentionally left empty.
+        }
+
         template<typename RationalNumberType>
         ToRationalNumberVisitor<RationalNumberType>::ToRationalNumberVisitor(ExpressionEvaluatorBase<RationalNumberType> const& evaluator) : ExpressionVisitor(), evaluator(evaluator) {
             // Intentionally left empty.
@@ -19,7 +24,13 @@ namespace storm {
         
         template<typename RationalNumberType>
         boost::any ToRationalNumberVisitor<RationalNumberType>::visit(IfThenElseExpression const& expression, boost::any const& data) {
-            bool conditionValue = evaluator.asBool(expression.getCondition());
+            bool conditionValue;
+            if (evaluator) {
+            	conditionValue = evaluator->asBool(expression.getCondition());
+            } else {
+            	// no evaluator, fall back to evaluateBool
+            	conditionValue = expression.getCondition()->evaluateAsBool();
+            }
             if (conditionValue) {
                 return expression.getThenExpression()->accept(*this, data);
             } else {
diff --git a/src/storm/storage/expressions/ToRationalNumberVisitor.h b/src/storm/storage/expressions/ToRationalNumberVisitor.h
index 89cb9f81f..55576e5ad 100644
--- a/src/storm/storage/expressions/ToRationalNumberVisitor.h
+++ b/src/storm/storage/expressions/ToRationalNumberVisitor.h
@@ -16,7 +16,8 @@ namespace storm {
         template<typename RationalNumberType>
         class ToRationalNumberVisitor : public ExpressionVisitor {
         public:
-            ToRationalNumberVisitor(ExpressionEvaluatorBase<RationalNumberType> const& evaluator);
+        	ToRationalNumberVisitor();
+        	ToRationalNumberVisitor(ExpressionEvaluatorBase<RationalNumberType> const& evaluator);
             
             RationalNumberType toRationalNumber(Expression const& expression);
             
@@ -36,8 +37,8 @@ namespace storm {
         private:
             std::unordered_map<storm::expressions::Variable, RationalNumberType> valueMapping;
 
-            // A reference to an expression evaluator (mainly for resolving the boolean condition in IfThenElse expressions)
-            ExpressionEvaluatorBase<RationalNumberType> const& evaluator;
+            // An optional reference to an expression evaluator (mainly for resolving the boolean condition in IfThenElse expressions)
+            boost::optional<ExpressionEvaluatorBase<RationalNumberType> const&> evaluator;
         };
     }
 }