diff --git a/js/src/jit-test/tests/jaeger/bug684824.js b/js/src/jit-test/tests/jaeger/bug684824.js
new file mode 100644
index 00000000000..9e8ad9c7ee7
--- /dev/null
+++ b/js/src/jit-test/tests/jaeger/bug684824.js
@@ -0,0 +1,7 @@
+
+function X(n) {
+    while ('' + (n--)) {
+        break;
+    }
+}
+X();
diff --git a/js/src/methodjit/Compiler.cpp b/js/src/methodjit/Compiler.cpp
index f1c15705272..fd2765a9c87 100644
--- a/js/src/methodjit/Compiler.cpp
+++ b/js/src/methodjit/Compiler.cpp
@@ -7216,7 +7216,12 @@ mjit::Compiler::updateJoinVarTypes()
             if (newv->slot < TotalSlots(script)) {
                 VarType &vt = a->varTypes[newv->slot];
                 vt.types = analysis->getValueTypes(newv->value);
-                vt.type = vt.types->getKnownTypeTag(cx);
+                JSValueType newType = vt.types->getKnownTypeTag(cx);
+                if (newType != vt.type) {
+                    FrameEntry *fe = frame.getSlotEntry(newv->slot);
+                    frame.forgetLoopReg(fe);
+                }
+                vt.type = newType;
             }
             newv++;
         }