diff -r e6cfbc3e2ed9 Modules/_tkinter.c
--- a/Modules/_tkinter.c	Thu Sep 04 09:53:16 2014 +0200
+++ b/Modules/_tkinter.c	Thu Sep 04 09:58:37 2014 +0200
@@ -1243,8 +1243,10 @@ Tkapp_Call(PyObject *selfptr, PyObject *
         Tkapp_CallEvent *ev;
         Tcl_Condition cond = NULL;
         PyObject *exc_type, *exc_value, *exc_tb;
-        if (!WaitForMainloop(self))
+        if (!WaitForMainloop(self)) {
+            assert(PyErr_Occurred());
             return NULL;
+        }
         ev = (Tkapp_CallEvent*)ckalloc(sizeof(Tkapp_CallEvent));
         ev->ev.proc = (Tcl_EventProc*)Tkapp_CallProc;
         ev->self = self;
@@ -1262,6 +1264,7 @@ Tkapp_Call(PyObject *selfptr, PyObject *
                 PyErr_Restore(exc_type, exc_value, exc_tb);
             else
                 PyErr_SetObject(Tkinter_TclError, exc_value);
+            assert(PyErr_Occurred());
         }
         Tcl_ConditionFinalize(&cond);
     }
@@ -1270,8 +1273,10 @@ Tkapp_Call(PyObject *selfptr, PyObject *
     {
 
         objv = Tkapp_CallArgs(args, objStore, &objc);
-        if (!objv)
+        if (!objv) {
+            assert(PyErr_Occurred());
             return NULL;
+        }
 
         ENTER_TCL
 
@@ -1281,8 +1286,10 @@ Tkapp_Call(PyObject *selfptr, PyObject *
 
         if (i == TCL_ERROR)
             Tkinter_Error(selfptr);
-        else
+        else {
             res = Tkapp_CallResult(self);
+            assert(res != NULL || PyErr_Occurred());
+        }
 
         LEAVE_OVERLAP_TCL