diff --git a/python/plugins/processing/core/parameters.py b/python/plugins/processing/core/parameters.py
index 7c9d897848ab..04e3a11de1ba 100644
--- a/python/plugins/processing/core/parameters.py
+++ b/python/plugins/processing/core/parameters.py
@@ -709,9 +709,10 @@ class ParameterString(Parameter):
     ESCAPED_NEWLINE = '\\n'
 
     def __init__(self, name='', description='', default=None, multiline=False,
-                 optional=False):
+                 optional=False, evaluateExpressions=False):
         Parameter.__init__(self, name, description, default, optional)
         self.multiline = parseBool(multiline)
+        self.evaluateExpressions = parseBool(evaluateExpressions)
 
     def setValue(self, obj):
         if obj is None:
diff --git a/python/plugins/processing/gui/AlgorithmDialog.py b/python/plugins/processing/gui/AlgorithmDialog.py
index 6b8480ebb25b..45c9408d2e65 100644
--- a/python/plugins/processing/gui/AlgorithmDialog.py
+++ b/python/plugins/processing/gui/AlgorithmDialog.py
@@ -169,14 +169,16 @@ def setParamValue(self, param, widget, alg=None):
             return param.setValue(widget.getValue())
         elif isinstance(param, ParameterString):
             if param.multiline:
-                return param.setValue(unicode(widget.toPlainText()))
+                text = unicode(widget.toPlainText())
             else:
                 text = widget.text()
+
+            if param.evaluateExpressions:
                 try:
                     text = self.evaluateExpression(text)
                 except:
                     pass
-                return param.setValue(text)
+            return param.setValue(text)
         elif isinstance(param, ParameterGeometryPredicate):
             return param.setValue(widget.value())
         else:
diff --git a/python/plugins/processing/tests/ParametersTest.py b/python/plugins/processing/tests/ParametersTest.py
index 83a2c0add1ce..30ad4580720f 100644
--- a/python/plugins/processing/tests/ParametersTest.py
+++ b/python/plugins/processing/tests/ParametersTest.py
@@ -36,7 +36,8 @@
                                         ParameterFixedTable,
                                         ParameterMultipleInput,
                                         ParameterNumber,
-                                        ParameterPoint)
+                                        ParameterPoint,
+                                        ParameterString)
 
 from qgis.core import (QgsRasterLayer,
                        QgsVectorLayer)
@@ -394,5 +395,28 @@ def testOptional(self):
         self.assertEqual(requiredParameter.value, 5)
 
 
+class ParameterStringTest(unittest.TestCase):
+
+    def testSetValue(self):
+        parameter = ParameterString('myName', 'myDescription')
+        self.assertTrue(parameter.setValue('test'))
+        self.assertEqual(parameter.value, 'test')
+
+    def testOptional(self):
+        optionalParameter = ParameterString('myName', 'myDesc', default='test', optional=True)
+        self.assertEqual(optionalParameter.value, 'test')
+        optionalParameter.setValue('check')
+        self.assertEqual(optionalParameter.value, 'check')
+        self.assertTrue(optionalParameter.setValue(None))
+        self.assertEqual(optionalParameter.value, None)
+
+        requiredParameter = ParameterCrs('myName', 'myDesc', default='test', optional=False)
+        self.assertEqual(requiredParameter.value, 'test')
+        requiredParameter.setValue('check')
+        self.assertEqual(requiredParameter.value, 'check')
+        self.assertFalse(requiredParameter.setValue(None))
+        self.assertEqual(requiredParameter.value, 'check')
+
+
 if __name__ == '__main__':
     unittest.main()