diff --git a/js/src/jit-test/tests/ion/bug913749.js b/js/src/jit-test/tests/ion/bug913749.js
new file mode 100644
index 00000000000..715483f7d3f
--- /dev/null
+++ b/js/src/jit-test/tests/ion/bug913749.js
@@ -0,0 +1,20 @@
+y = Float32Array(11);
+x = [];
+
+Object.defineProperty(x, 18, {
+    get: (function() {
+        y.length;
+    }),
+});
+this.toSource();
+
+y = undefined;
+
+for (var i = 0; i < 3; i++) {
+    try {
+	x.toString();
+	assertEq(0, 1);
+    } catch (e) {
+	assertEq(e.message, "y is undefined");
+    }
+}
diff --git a/js/src/jit/MIR.h b/js/src/jit/MIR.h
index 905aa71f159..6274a8257c7 100644
--- a/js/src/jit/MIR.h
+++ b/js/src/jit/MIR.h
@@ -2197,7 +2197,7 @@ class MUnbox : public MUnaryInstruction, public BoxInputsPolicy
         setResultTypeSet(ins->resultTypeSet());
         setMovable();
 
-        if (mode_ == TypeBarrier)
+        if (mode_ == TypeBarrier || mode_ == Fallible)
             setGuard();
 
         bailoutKind_ = kind;