diff --git a/src/serialize.cc b/src/serialize.cc
index d5d06a0677..0b39d25642 100644
--- a/src/serialize.cc
+++ b/src/serialize.cc
@@ -480,16 +480,14 @@ ExternalReferenceDecoder::~ExternalReferenceDecoder() {
 RootIndexMap::RootIndexMap(Isolate* isolate) {
   map_ = new HashMap(HashMap::PointersMatch);
   Object** root_array = isolate->heap()->roots_array_start();
-  for (int i = 0; i < Heap::kStrongRootListLength; i++) {
+  for (uint32_t i = 0; i < Heap::kStrongRootListLength; i++) {
     Object* root = root_array[i];
     if (root->IsHeapObject() && !isolate->heap()->InNewSpace(root)) {
       HeapObject* heap_object = HeapObject::cast(root);
-      if (LookupEntry(map_, heap_object, false) != NULL) {
-        // Some root values are initialized to the empty FixedArray();
-        // Do not add them to the map.
-        // TODO(yangguo): This assert is not true. Some roots like
-        // instanceof_cache_answer can be e.g. null.
-        // DCHECK_EQ(isolate->heap()->empty_fixed_array(), heap_object);
+      HashMap::Entry* entry = LookupEntry(map_, heap_object, false);
+      if (entry != NULL) {
+        // Some are initialized to a previous value in the root list.
+        DCHECK_LT(GetValue(entry), i);
       } else {
         SetValue(LookupEntry(map_, heap_object, true), i);
       }
diff --git a/src/serialize.h b/src/serialize.h
index 6817c7efe6..06ed9fc4f2 100644
--- a/src/serialize.h
+++ b/src/serialize.h
@@ -150,8 +150,8 @@ class AddressMapBase {
     return static_cast<uint32_t>(reinterpret_cast<intptr_t>(entry->value));
   }
 
-  static HashMap::Entry* LookupEntry(HashMap* map, HeapObject* obj,
-                                     bool insert) {
+  inline static HashMap::Entry* LookupEntry(HashMap* map, HeapObject* obj,
+                                            bool insert) {
     return map->Lookup(Key(obj), Hash(obj), insert);
   }
 
@@ -195,9 +195,9 @@ class PartialCacheIndexMap : public AddressMapBase {
   // Lookup object in the map. Return its index if found, or create
   // a new entry with new_index as value, and return kInvalidIndex.
   int LookupOrInsert(HeapObject* obj, int new_index) {
-    HashMap::Entry* entry = LookupEntry(&map_, obj, true);
-    if (entry->value != NULL) return GetValue(entry);
-    SetValue(entry, static_cast<uint32_t>(new_index));
+    HashMap::Entry* entry = LookupEntry(&map_, obj, false);
+    if (entry != NULL) return GetValue(entry);
+    SetValue(LookupEntry(&map_, obj, true), static_cast<uint32_t>(new_index));
     return kInvalidIndex;
   }