diff --git a/LOG b/LOG
index 89a15ea1a7..32eb516dc3 100644
--- a/LOG
+++ b/LOG
@@ -892,3 +892,6 @@
     schlib.c, prim.c, externs.h
     mats/foreign4.c, mats/foreign.ms mats/Mf-*
     foreign.stex, release_notes.stex
+- reworked the S_call_help/S_return CCHAIN handling to fix a bug in which
+  the signal handler could trip over the NULL jumpbuf in a CCHAIN record.  
+    schlib.c
diff --git a/c/schlib.c b/c/schlib.c
index be9259ad45..f9ea4b6ad4 100644
--- a/c/schlib.c
+++ b/c/schlib.c
@@ -219,14 +219,17 @@ void S_call_help(tc, singlep, lock_ts) ptr tc; IBOOL singlep; IBOOL lock_ts; {
     jb = CREATEJMPBUF();
     if (jb == NULL)
       S_error_abort("unable to allocate memory for jump buffer");
-    FRAME(tc, -1) = CCHAIN(tc) = Scons(Scons(jb, code), CCHAIN(tc));
     if (lock_ts) {
       /* Lock a code object passed in TS, which is a more immediate
          caller whose return address is on the C stack */
       Slock_object(TS(tc));
-      CCHAIN(tc) = Scons(Scons(NULL, TS(tc)), CCHAIN(tc));
+      CCHAIN(tc) = Scons(Scons(jb, Scons(code,TS(tc))), CCHAIN(tc));
+    } else {
+      CCHAIN(tc) = Scons(Scons(jb, Scons(code,Sfalse)), CCHAIN(tc));
     }
 
+    FRAME(tc, -1) = CCHAIN(tc);
+
     switch (SETJMP(jb)) {
         case 0: /* first time */
             S_generic_invoke(tc, S_G.invoke_code_object);
@@ -268,11 +271,10 @@ void S_call_any_results() {
     S_call_help(tc, 0, 1);
 }
 
-/* cchain = ((jb . co) ...) */
+/* cchain = ((jb . (co . maybe-co)) ...) */
 void S_return() {
     ptr tc = get_thread_context();
     ptr xp, yp;
-    void *jb;
 
     SFP(tc) = (ptr)((ptr *)SFP(tc) - 2);
 
@@ -286,11 +288,11 @@ void S_return() {
 
   /* error checks are done; now unlock affected code objects */
     for (xp = CCHAIN(tc); ; xp = Scdr(xp)) {
-        Sunlock_object(CDAR(xp));
+        ptr p = CDAR(xp);
+        Sunlock_object(Scar(p));
+        if (Scdr(p) != Sfalse) Sunlock_object(Scdr(p));
         if (xp == yp) break;
-        jb = CAAR(xp);
-        if (jb != NULL)
-          FREEJMPBUF(jb);
+        FREEJMPBUF(CAAR(xp));
     }
 
   /* reset cchain and return via longjmp */