diff --git a/DSView/pv/sigsession.cpp b/DSView/pv/sigsession.cpp
index f85deacb..69ed2aaa 100644
--- a/DSView/pv/sigsession.cpp
+++ b/DSView/pv/sigsession.cpp
@@ -1480,8 +1480,9 @@ namespace pv
         assert(dsoSig1);
         assert(dsoSig2);
 
-        auto math_stack = new data::MathStack(this, dsoSig1, dsoSig2, type);
         DESTROY_OBJECT(_math_trace);
+
+        auto math_stack = new data::MathStack(this, dsoSig1, dsoSig2, type);        
         _math_trace = new view::MathTrace(enable, math_stack, dsoSig1, dsoSig2);
 
         if (_math_trace && _math_trace->enabled())
@@ -2079,7 +2080,9 @@ namespace pv
     }
 
     void SigSession::clear_signals()
-    {
+    {   
+        DESTROY_OBJECT(_math_trace);
+        
         for (int i=0; i< (int)_signals.size(); i++)
         {
             auto *p = _signals[i];
diff --git a/DSView/pv/view/mathtrace.cpp b/DSView/pv/view/mathtrace.cpp
index 836f0601..5c394ec6 100644
--- a/DSView/pv/view/mathtrace.cpp
+++ b/DSView/pv/view/mathtrace.cpp
@@ -76,11 +76,13 @@ void MathTrace::set_enable(bool enable)
 
 int MathTrace::src1()
 {
+    assert(_dsoSig1);
     return _dsoSig1->get_index();
 }
 
 int MathTrace::src2()
-{
+{   
+    assert(_dsoSig2);
     return _dsoSig2->get_index();
 }