diff -r 1addc5d2c246 Objects/genobject.c
--- a/Objects/genobject.c	Tue Nov 22 23:04:39 2016 +0100
+++ b/Objects/genobject.c	Thu Nov 24 00:06:47 2016 +0100
@@ -354,8 +354,15 @@ PyObject *
     if (f && f->f_stacktop) {
         PyObject *bytecode = f->f_code->co_code;
         unsigned char *code = (unsigned char *)PyBytes_AS_STRING(bytecode);
+        int next_instr;
 
-        if (code[f->f_lasti + sizeof(_Py_CODEUNIT)] != YIELD_FROM)
+        if (f->f_lasti >= 0) {
+            next_instr = f->f_lasti + sizeof(_Py_CODEUNIT);
+        }
+        else {
+            next_instr = 0;
+        }
+        if (code[next_instr] != YIELD_FROM)
             return NULL;
         yf = f->f_stacktop[-1];
         Py_INCREF(yf);