--- a/js/src/vm/EnvironmentObject-inl.h
+++ b/js/src/vm/EnvironmentObject-inl.h
@@ -12,18 +12,21 @@
 #include "vm/JSObject-inl.h"
 #include "vm/TypeInference-inl.h"
 
 namespace js {
 
 inline LexicalEnvironmentObject&
 NearestEnclosingExtensibleLexicalEnvironment(JSObject* env)
 {
-    while (!IsExtensibleLexicalEnvironment(env))
+    MOZ_ASSERT(env);
+    while (!IsExtensibleLexicalEnvironment(env)) {
         env = env->enclosingEnvironment();
+        MOZ_ASSERT(env);
+    }
     return env->as<LexicalEnvironmentObject>();
 }
 
 inline void
 EnvironmentObject::setAliasedBinding(JSContext* cx, uint32_t slot, PropertyName* name,
                                      const Value& v)
 {
     if (isSingleton()) {