--- a/js/src/jit-test/tests/basic/expression-autopsy.js
+++ b/js/src/jit-test/tests/basic/expression-autopsy.js
@@ -41,25 +41,39 @@ function check(expr, expected=expr) {
             // Let block
             Function("let (o, undef) { " + statement + " }"),
             // Let block with some other variables
             Function("var v1, v2; let (o, undef) { " + statement + " }"),
             // Shadowed let block
             Function("o", "undef", "let (o, undef) { " + statement + " }"),
             // Let in a switch
             Function("var x = 4; switch (x) { case 4: let o, undef;" + statement + "\ncase 6: break;}"),
-            // Let in for-in
-            Function("var undef, o; for (let z in [1, 2]) { " + statement + " }"),
             // The more lets the merrier
             Function("let (x=4, y=5) { x + y; }\nlet (a, b, c) { a + b - c; }\nlet (o, undef) {" + statement + " }"),
             // Let destructuring
             Function("o", "undef", "let ([] = 4) {} let (o, undef) { " + statement + " }"),
             // Try-catch blocks
             Function("o", "undef", "try { let q = 4; try { let p = 4; } catch (e) {} } catch (e) {} let (o, undef) { " + statement + " }")
         ];
+
+        try {
+            // Let in for-in
+            check_one(expected,
+                      Function("var undef, o; for (let z in [1, 2]) { " + statement + " }"),
+                      err);
+        } catch (ex) {
+            // Bug 831120.  See bug 942804 comment 5.
+            if (expected == 'undef' && err == ' is undefined')
+                check_one(expected + end,
+                          Function("var undef, o; for (let z in [1, 2]) { " + statement + " }"),
+                          err);
+            else
+                throw ex;
+        }
+
         for (var f of cases) {
             check_one(expected, f, err);
         }
     }
 }
 
 check("undef");
 check("o.b");

--- a/js/src/jit/IonBuilder.cpp
+++ b/js/src/jit/IonBuilder.cpp
@@ -1763,16 +1763,19 @@ IonBuilder::inspectOpcode(JSOp op)
       case JSOP_SETRVAL:
         JS_ASSERT(!script()->noScriptRval());
         current->setSlot(info().returnValueSlot(), current->pop());
         return true;
 
       case JSOP_INSTANCEOF:
         return jsop_instanceof();
 
+      case JSOP_DEBUGLEAVEBLOCK:
+        return true;
+
       default:
 #ifdef DEBUG
         return abort("Unsupported opcode: %s (line %d)", js_CodeName[op], info().lineno(pc));
 #else
         return abort("Unsupported opcode: %d (line %d)", op, info().lineno(pc));
 #endif
     }
 }