diff --git a/python/plugins/processing/algs/qgis/FieldsCalculator.py b/python/plugins/processing/algs/qgis/FieldsCalculator.py
index 61605e42e69..2c4b5f48c1a 100644
--- a/python/plugins/processing/algs/qgis/FieldsCalculator.py
+++ b/python/plugins/processing/algs/qgis/FieldsCalculator.py
@@ -147,15 +147,13 @@ class FieldsCalculator(GeoAlgorithm):
 
     def checkParameterValuesBeforeExecuting(self):
         newField = self.getParameterValue(self.NEW_FIELD)
-        fieldName = self.getParameterValue(self.FIELD_NAME)
+        fieldName = self.getParameterValue(self.FIELD_NAME).strip()
         if newField and len(fieldName) == 0:
-            raise GeoAlgorithmExecutionException(
-                self.tr('Field name is not set. Please enter a field name'))
+            return self.tr('Field name is not set. Please enter a field name')
 
-        outputName = self.getOutputValue(self.OUTPUT_LAYER)
+        outputName = self.getOutputValue(self.OUTPUT_LAYER).strip()
         if outputName == '':
-            raise GeoAlgorithmExecutionException(
-                self.tr('Output is not set. Please specify valid filename'))
+            return self.tr('Output is not set. Please specify valid filename')
 
     def getCustomParametersDialog(self):
         return FieldsCalculatorDialog(self)
diff --git a/python/plugins/processing/algs/qgis/ui/FieldsCalculatorDialog.py b/python/plugins/processing/algs/qgis/ui/FieldsCalculatorDialog.py
index 06a76bbc819..1cb3a1352c3 100644
--- a/python/plugins/processing/algs/qgis/ui/FieldsCalculatorDialog.py
+++ b/python/plugins/processing/algs/qgis/ui/FieldsCalculatorDialog.py
@@ -182,6 +182,12 @@ class FieldsCalculatorDialog(QDialog, Ui_FieldsCalculator):
         self.alg.setParameterValue('FORMULA', self.builder.expressionText())
         self.alg.setOutputValue('OUTPUT_LAYER',
                 self.leOutputFile.text())
+
+        msg = self.alg.checkParameterValuesBeforeExecuting()
+        if msg:
+            QMessageBox.warning(
+                self, self.tr('Unable to execute algorithm'), msg)
+            return False
         return True
 
     def accept(self):
@@ -199,11 +205,6 @@ class FieldsCalculatorDialog(QDialog, Ui_FieldsCalculator):
                                            not keepOpen)
                 if not keepOpen:
                     QDialog.reject(self)
-            else:
-                QMessageBox.critical(self,
-                                     self.tr('Unable to execute algorithm'),
-                                     self.tr('Wrong or missing parameter '
-                                             'values'))
         finally:
             QApplication.restoreOverrideCursor()