diff --git a/src/build/toolset.py b/src/build/toolset.py
index d7b121a2a..84fa767f0 100644
--- a/src/build/toolset.py
+++ b/src/build/toolset.py
@@ -117,8 +117,8 @@ def flags(rule_or_module, variable_name, condition, values = []):
                           is specified, then the value of 'feature' 
                           will be added.
     """
-    caller = bjam.caller()[:-1]
-    if not '.' in rule_or_module and caller.startswith("Jamfile"):
+    caller = bjam.caller()
+    if not '.' in rule_or_module and caller and caller[:-1].startswith("Jamfile"):
         # Unqualified rule name, used inside Jamfile. Most likely used with
         # 'make' or 'notfile' rules. This prevents setting flags on the entire
         # Jamfile module (this will be considered as rule), but who cares?
diff --git a/src/engine/builtins.c b/src/engine/builtins.c
index dc723e19b..449823973 100644
--- a/src/engine/builtins.c
+++ b/src/engine/builtins.c
@@ -2140,9 +2140,9 @@ PyObject * bjam_backtrace( PyObject * self, PyObject * args )
 
 PyObject * bjam_caller( PyObject * self, PyObject * args )
 {
-    PyObject *result = PyString_FromString(
-        frame_before_python_call->prev->module->name);
-    return result;
+    if ( !frame_before_python_call )
+        Py_RETURN_NONE;
+    return PyString_FromString(frame_before_python_call->prev->module->name);
 }
 
 #endif  /* #ifdef HAVE_PYTHON */
diff --git a/src/engine/compile.c b/src/engine/compile.c
index 04783f1c8..dd8c7f22d 100644
--- a/src/engine/compile.c
+++ b/src/engine/compile.c
@@ -865,6 +865,7 @@ call_python_function(RULE* r, FRAME* frame)
 
     frame_before_python_call = frame;
     py_result = PyObject_Call( r->python_function, arguments, kw );
+    frame_before_python_call = NULL;
     Py_DECREF(arguments);
     Py_XDECREF(kw);
     if ( py_result != NULL )