diff --git a/src/core/qgsexpressionfieldbuffer.cpp b/src/core/qgsexpressionfieldbuffer.cpp
index c6f2b38b5b9..bb54cb48fb2 100644
--- a/src/core/qgsexpressionfieldbuffer.cpp
+++ b/src/core/qgsexpressionfieldbuffer.cpp
@@ -36,6 +36,7 @@ void QgsExpressionFieldBuffer::removeExpression( int index )
 void QgsExpressionFieldBuffer::updateExpression( int index, const QString& exp )
 {
   mExpressions[index].expression = exp;
+  mExpressions[index].cachedExpression = QgsExpression( exp );
 }
 
 void QgsExpressionFieldBuffer::writeXml( QDomNode& layerNode, QDomDocument& document ) const
diff --git a/src/core/qgsexpressionfieldbuffer.h b/src/core/qgsexpressionfieldbuffer.h
index f94fb5a14d5..69feb960709 100644
--- a/src/core/qgsexpressionfieldbuffer.h
+++ b/src/core/qgsexpressionfieldbuffer.h
@@ -23,6 +23,7 @@
 #include <QDomNode>
 
 #include "qgsfield.h"
+#include "qgsexpression.h"
 
 /**
  * Buffers information about expression fields for a vector layer.
@@ -34,10 +35,14 @@ class CORE_EXPORT QgsExpressionFieldBuffer
   public:
     typedef struct ExpressionField
     {
-      ExpressionField() {}
-      ExpressionField( const QString& exp, const QgsField& fld ) : expression( exp ), field( fld ) {}
+      ExpressionField() : cachedExpression( expression ) {}
+      ExpressionField( const QString& exp, const QgsField& fld ) : expression( exp ), cachedExpression( exp ), field( fld ) {}
 
+      /**
+       * @deprecated use cachedExpression instead
+       */
       QString expression;
+      QgsExpression cachedExpression;
       QgsField field;
     } ExpressionField;
 
diff --git a/src/core/qgsvectorlayerfeatureiterator.cpp b/src/core/qgsvectorlayerfeatureiterator.cpp
index 550443586a8..80bb5aecab0 100644
--- a/src/core/qgsvectorlayerfeatureiterator.cpp
+++ b/src/core/qgsvectorlayerfeatureiterator.cpp
@@ -530,7 +530,7 @@ void QgsVectorLayerFeatureIterator::prepareExpressions()
            || mRequest.subsetOfAttributes().contains( i ) )
       {
         int oi = mSource->mFields.fieldOriginIndex( i );
-        QgsExpression* exp = new QgsExpression( exps[oi].expression );
+        QgsExpression* exp = new QgsExpression( exps[oi].cachedExpression );
         exp->prepare( mExpressionContext.data() );
         mExpressionFieldInfo.insert( i, exp );
 
diff --git a/tests/src/python/test_qgsvectorlayer.py b/tests/src/python/test_qgsvectorlayer.py
index c6ebd6c7edb..86c3153a0d8 100644
--- a/tests/src/python/test_qgsvectorlayer.py
+++ b/tests/src/python/test_qgsvectorlayer.py
@@ -965,16 +965,16 @@ class TestQgsVectorLayer(TestCase):
 
         idx = layer.addExpressionField('5', QgsField('test', QVariant.LongLong))
 
-        assert(layer.getFeatures().next()[idx] == 5)
-        assert(layer.pendingFields().count() == cnt + 1)
+        self.assertEquals(layer.getFeatures().next()[idx], 5)
+        self.assertEquals(layer.pendingFields().count(), cnt + 1)
 
         layer.updateExpressionField(idx, '9')
 
-        assert(layer.getFeatures().next()[idx] == 9)
+        self.assertEquals(layer.getFeatures().next()[idx], 9)
 
         layer.removeExpressionField(idx)
 
-        assert(layer.pendingFields().count() == cnt)
+        self.assertEquals(layer.pendingFields().count(), cnt)
 
     def test_ExpressionFilter(self):
         layer = createLayerWithOnePoint()