diff --git a/vm/mach_signal.c b/vm/mach_signal.c
index 118fc7044c..bfffa4cee0 100644
--- a/vm/mach_signal.c
+++ b/vm/mach_signal.c
@@ -37,6 +37,8 @@ static void call_fault_handler(exception_type_t exception,
 	else
 		signal_callstack_top = NULL;
 
+	MACH_STACK_POINTER(thread_state) = fix_stack_pointer(MACH_STACK_POINTER(thread_state));
+
 	/* Now we point the program counter at the right handler function. */
 	if(exception == EXC_BAD_ACCESS)
 	{
diff --git a/vm/os-macosx-ppc.h b/vm/os-macosx-ppc.h
index b6c2cc6859..640aeb796d 100644
--- a/vm/os-macosx-ppc.h
+++ b/vm/os-macosx-ppc.h
@@ -30,3 +30,8 @@ Modified for Factor by Slava Pestov */
 	#define UAP_PROGRAM_COUNTER(ucontext) \
 		MACH_PROGRAM_COUNTER(&(((ucontext_t *)(ucontext))->uc_mcontext->ss))
 #endif
+
+INLINE CELL fix_stack_pointer(CELL sp)
+{
+	return sp;
+}
diff --git a/vm/os-macosx-x86.32.h b/vm/os-macosx-x86.32.h
index b77a0c7742..d5e5827a5c 100644
--- a/vm/os-macosx-x86.32.h
+++ b/vm/os-macosx-x86.32.h
@@ -28,3 +28,8 @@ Modified for Factor by Slava Pestov */
 	#define UAP_PROGRAM_COUNTER(ucontext) \
 		MACH_PROGRAM_COUNTER(&(((ucontext_t *)(ucontext))->uc_mcontext->ss))	
 #endif
+
+INLINE CELL fix_stack_pointer(CELL sp)
+{
+	return ((sp + 4) & ~15) - 4;
+}