diff --git a/js/src/methodjit/MonoIC.cpp b/js/src/methodjit/MonoIC.cpp
index 336de5b7a44..032073e8a90 100644
--- a/js/src/methodjit/MonoIC.cpp
+++ b/js/src/methodjit/MonoIC.cpp
@@ -142,6 +142,7 @@ ic::SetGlobalName(VMFrame &f, uint32 index)
     JSScopeProperty *sprop = scope->lookup(id);
     if (!sprop ||
         !sprop->hasDefaultGetterOrIsMethod() ||
+        !sprop->writable() ||
         !SPROP_HAS_VALID_SLOT(sprop, scope))
     {
         JS_UNLOCK_SCOPE(f.cx, scope);
diff --git a/js/src/trace-test/tests/jaeger/bug584647.js b/js/src/trace-test/tests/jaeger/bug584647.js
new file mode 100644
index 00000000000..7f0e6598481
--- /dev/null
+++ b/js/src/trace-test/tests/jaeger/bug584647.js
@@ -0,0 +1,2 @@
+(function(){for(j=0;j<3;++j)NaN=42})();
+assertEq(NaN != NaN, true);