diff --git a/base/lib/iscore/tools/TreeNode.hpp b/base/lib/iscore/tools/TreeNode.hpp
index a217dd04a4..b39a661038 100644
--- a/base/lib/iscore/tools/TreeNode.hpp
+++ b/base/lib/iscore/tools/TreeNode.hpp
@@ -63,6 +63,7 @@ class TreeNode : public DataType
             static_cast<DataType&>(*this) = static_cast<const DataType&>(source);
 
             qDeleteAll(m_children);
+            m_children.clear();
             for(const auto& child : source.children())
             {
                 this->addChild(new TreeNode{*child, this});
diff --git a/base/plugins/iscore-lib-state/State/Expression.hpp b/base/plugins/iscore-lib-state/State/Expression.hpp
index 60b1b9562e..01add03dac 100644
--- a/base/plugins/iscore-lib-state/State/Expression.hpp
+++ b/base/plugins/iscore-lib-state/State/Expression.hpp
@@ -35,7 +35,15 @@ struct ExprData : public VariantBasedNode<Relation, BinaryOperator, UnaryOperato
 
 }
 using iscore::ExprData;
+
 template<>
+/**
+ * @brief The TreeNode<ExprData> class
+ *
+ * This class is specialized from TreeNode<T>
+ * because we want to have an additional check :
+ * a node is a leaf iff a node is a iscore::Relation
+ */
 class TreeNode<ExprData> : public ExprData
 {
         ISCORE_SERIALIZE_FRIENDS(TreeNode<ExprData>, DataStream)
diff --git a/base/plugins/iscore-lib-state/State/ExpressionSerialization.cpp b/base/plugins/iscore-lib-state/State/ExpressionSerialization.cpp
index ce1fa6537d..a95cea3b0f 100644
--- a/base/plugins/iscore-lib-state/State/ExpressionSerialization.cpp
+++ b/base/plugins/iscore-lib-state/State/ExpressionSerialization.cpp
@@ -58,28 +58,28 @@ template<> class TypeToName<iscore::UnaryOperator>
 
 
 template<>
-void Visitor<Reader<DataStream>>::readFrom(const ExprData& state)
+void Visitor<Reader<DataStream>>::readFrom(const ExprData& expr)
 {
-    readFrom(state.m_data);
+    readFrom(expr.m_data);
     insertDelimiter();
 }
 
 template<>
-void Visitor<Reader<JSONObject>>::readFrom(const ExprData& state)
+void Visitor<Reader<JSONObject>>::readFrom(const ExprData& expr)
 {
-    readFrom(state.m_data);
+    readFrom(expr.m_data);
 }
 
 template<>
-void Visitor<Writer<DataStream>>::writeTo(ExprData& state)
+void Visitor<Writer<DataStream>>::writeTo(ExprData& expr)
 {
-    writeTo(state.m_data);
+    writeTo(expr.m_data);
     checkDelimiter();
 }
 
 template<>
-void Visitor<Writer<JSONObject>>::writeTo(ExprData& state)
+void Visitor<Writer<JSONObject>>::writeTo(ExprData& expr)
 {
-    writeTo(state.m_data);
+    writeTo(expr.m_data);
 }
 
diff --git a/base/plugins/iscore-plugin-ossia/DocumentPlugin/OSSIAScenarioElement.cpp b/base/plugins/iscore-plugin-ossia/DocumentPlugin/OSSIAScenarioElement.cpp
index eab81689eb..3ebe006bbb 100644
--- a/base/plugins/iscore-plugin-ossia/DocumentPlugin/OSSIAScenarioElement.cpp
+++ b/base/plugins/iscore-plugin-ossia/DocumentPlugin/OSSIAScenarioElement.cpp
@@ -201,7 +201,15 @@ void OSSIAScenarioElement::on_eventCreated(const EventModel& const_ev)
 
     connect(&ev, &EventModel::conditionChanged,
             this, [=] (const iscore::Condition& c) {
-        ossia_ev->setExpression(iscore::convert::expression(c, m_deviceList));
+        try {
+            auto expr = iscore::convert::expression(c, m_deviceList);
+
+            ossia_ev->setExpression(expr);
+        }
+        catch(std::exception& e)
+        {
+            qDebug() << e.what();
+        }
     });
 
     // Create the mapping object
diff --git a/base/plugins/iscore-plugin-ossia/iscore2OSSIA.cpp b/base/plugins/iscore-plugin-ossia/iscore2OSSIA.cpp
index 909a7b6af0..3219180fc0 100644
--- a/base/plugins/iscore-plugin-ossia/iscore2OSSIA.cpp
+++ b/base/plugins/iscore-plugin-ossia/iscore2OSSIA.cpp
@@ -14,6 +14,25 @@
 
 #include "Protocols/OSSIADevice.hpp"
 #include <QMap>
+
+
+class NodeNotFoundException : public std::exception
+{
+        const iscore::Address& m_addr;
+    public:
+        NodeNotFoundException(const iscore::Address& n):
+            m_addr{n}
+        {
+
+        }
+
+        const char* what() const noexcept override
+        {
+            return QString("Address: %1 not found in actual tree.")
+                    .arg(m_addr.toString()).toLatin1().constData();
+        }
+};
+
 namespace iscore
 {
 namespace convert
@@ -372,6 +391,8 @@ std::shared_ptr<OSSIA::State> state(
     return ossia_state;
 }
 
+
+
 OSSIA::Value* expressionOperand(
         const iscore::RelationMember& relm,
         const DeviceList& devlist)
@@ -383,7 +404,9 @@ OSSIA::Value* expressionOperand(
         {
             const auto& addr = get<iscore::Address>(relm);
             if(!devlist.hasDevice(addr.device))
-                return nullptr;
+            {
+                throw NodeNotFoundException(addr);
+            }
 
             auto& device = devlist.device(addr.device);
 
@@ -396,12 +419,12 @@ OSSIA::Value* expressionOperand(
                 }
                 else
                 {
-                    return nullptr;
+                    throw NodeNotFoundException(addr);
                 }
             }
             else
             {
-                return nullptr;
+                throw NodeNotFoundException(addr);
             }
 
             break;