diff --git a/apps/vaporgui/PythonVariables.cpp b/apps/vaporgui/PythonVariables.cpp
index 762fdb37ca..d312ff113b 100644
--- a/apps/vaporgui/PythonVariables.cpp
+++ b/apps/vaporgui/PythonVariables.cpp
@@ -107,7 +107,10 @@ PythonVariables::~PythonVariables()
 
 void PythonVariables::Update(bool internalUpdate)
 {
-    if ((_scriptName == "") || (_dataMgrName == "")) {
+    VAPoR::DataStatus * dataStatus = _controlExec->GetDataStatus();
+    std::vector<string> dataMgrNames = dataStatus->GetDataMgrNames();
+
+    if ((_scriptName == "") || (_dataMgrName == "") || ((std::find(dataMgrNames.begin(), dataMgrNames.end(), _dataMgrName) == dataMgrNames.end()))) {
         _reset();
         _setGUIEnabled(false);
     } else {
diff --git a/include/vapor/CalcEngineMgr.h b/include/vapor/CalcEngineMgr.h
index 34b5b3e88c..315f46961a 100644
--- a/include/vapor/CalcEngineMgr.h
+++ b/include/vapor/CalcEngineMgr.h
@@ -50,6 +50,11 @@ class RENDER_API CalcEngineMgr : public Wasp::MyBase {
     //
     void ReinitFromState() { _sync(); }
 
+    //! Remove all functions added with AddFunction()
+    //!
+    //! \sa AddFunction()
+    void Clean() { _clean(); }
+
 private:
     const DataStatus *_dataStatus;
     const ParamsMgr * _paramsMgr;
diff --git a/lib/render/ControlExecutive.cpp b/lib/render/ControlExecutive.cpp
index 721d7f9898..8362017df7 100644
--- a/lib/render/ControlExecutive.cpp
+++ b/lib/render/ControlExecutive.cpp
@@ -458,6 +458,7 @@ int ControlExec::OpenData(const std::vector<string> &files, const std::vector<st
     // new data manager
     //
     if (_dataStatus->GetDataMgr(dataSetName)) {
+        _calcEngineMgr->Clean();
         _dataStatus->Close(dataSetName);
 
         vector<string> vizNames = _paramsMgr->GetVisualizerNames();
@@ -488,6 +489,7 @@ int ControlExec::OpenData(const std::vector<string> &files, const std::vector<st
     for (int i = 0; i < appRenderParams.size(); i++) {
         int rc = appRenderParams[i]->Initialize();
         if (rc < 0) {
+            _calcEngineMgr->Clean();
             _dataStatus->Close(dataSetName);
             _paramsMgr->RemoveDataMgr(dataSetName);
             SetErrMsg("Failure to initialize application renderer \"%s\"", appRenderParams[i]->GetName().c_str());
@@ -530,11 +532,7 @@ void ControlExec::CloseData(string dataSetName)
 
     _paramsMgr->RemoveDataMgr(dataSetName);
 
-    // Rebuild the calculation engine from params database after removing
-    // the data set from the params database.
-    //
-    _calcEngineMgr->ReinitFromState();
-
+    _calcEngineMgr->Clean();
     _dataStatus->Close(dataSetName);
 
     UndoRedoClear();