diff --git a/js/src/jit/IonAnalysis.cpp b/js/src/jit/IonAnalysis.cpp
index f6ad33c1d84..999c38c2eae 100644
--- a/js/src/jit/IonAnalysis.cpp
+++ b/js/src/jit/IonAnalysis.cpp
@@ -1576,11 +1576,16 @@ TryEliminateTypeBarrierFromTest(MTypeBarrier *barrier, bool filtersNull, bool fi
 
     // Disregard the possible unbox added before the Typebarrier for checking.
     MDefinition *input = barrier->input();
-    if (input->isUnbox() && input->toUnbox()->mode() != MUnbox::Fallible)
-        input = input->toUnbox()->input();
+    MUnbox *inputUnbox = nullptr;
+    if (input->isUnbox() && input->toUnbox()->mode() != MUnbox::Fallible) {
+        inputUnbox = input->toUnbox();
+        input = inputUnbox->input();
+    }
 
     if (test->getOperand(0) == input && direction == TRUE_BRANCH) {
         *eliminated = true;
+        if (inputUnbox)
+            inputUnbox->makeInfallible();
         barrier->replaceAllUsesWith(barrier->input());
         return;
     }
@@ -1614,6 +1619,8 @@ TryEliminateTypeBarrierFromTest(MTypeBarrier *barrier, bool filtersNull, bool fi
     }
 
     *eliminated = true;
+    if (inputUnbox)
+        inputUnbox->makeInfallible();
     barrier->replaceAllUsesWith(barrier->input());
 }
 
diff --git a/js/src/jit/MIR.h b/js/src/jit/MIR.h
index 849a6571620..25b8ca986bb 100644
--- a/js/src/jit/MIR.h
+++ b/js/src/jit/MIR.h
@@ -2373,6 +2373,11 @@ class MUnbox : public MUnaryInstruction, public BoxInputsPolicy
         return AliasSet::None();
     }
     void printOpcode(FILE *fp) const;
+    void makeInfallible() {
+        // Should only be called if we're already Infallible or TypeBarrier
+        JS_ASSERT(mode() != Fallible);
+        mode_ = Infallible;
+    }
 };
 
 class MGuardObject : public MUnaryInstruction, public SingleObjectPolicy