diff --git a/Include/frameobject.h b/Include/frameobject.h
index 1e3a01e..cce598b 100644
--- a/Include/frameobject.h
+++ b/Include/frameobject.h
@@ -26,7 +26,16 @@
        to the current stack top. */
     PyObject **f_stacktop;
     PyObject *f_trace;		/* Trace function */
+
+    /* If an exception is raised in this frame, the next three are used to
+     * record the exception info (if any) originally in the thread state.  See
+     * comments before set_exc_info() -- it's not obvious.
+     * Invariant:  if _type is NULL, then so are _value and _traceback.
+     * Desired invariant:  all three are NULL, or all three are non-NULL.  That
+     * one isn't currently true, but "should be".
+     */
     PyObject *f_exc_type, *f_exc_value, *f_exc_traceback;
+
     PyThreadState *f_tstate;
     int f_lasti;		/* Last instruction if called */
     /* As of 2.3 f_lineno is only valid when tracing is active (i.e. when