diff --git a/xpcom/glue/nsBaseHashtable.h b/xpcom/glue/nsBaseHashtable.h
index 6ab8590865a..a1a8146ba41 100644
--- a/xpcom/glue/nsBaseHashtable.h
+++ b/xpcom/glue/nsBaseHashtable.h
@@ -274,14 +274,15 @@ public:
   size_t SizeOfExcludingThis(SizeOfEntryExcludingThisFun sizeOfEntryExcludingThis,
                              nsMallocSizeOfFun mallocSizeOf, void *userArg = nsnull)
   {
-    if (IsInitialized()) {
-      s_SizeOfArgs args = { sizeOfEntryExcludingThis, userArg };
-      return PL_DHashTableSizeOfExcludingThis(&this->mTable,
-                                              s_SizeOfStub,
-                                              mallocSizeOf,
-                                              &args);
+    if (!IsInitialized()) {
+      return 0;
     }
-    return 0;
+    if (sizeOfEntryExcludingThis) {
+      s_SizeOfArgs args = { sizeOfEntryExcludingThis, userArg };
+      return PL_DHashTableSizeOfExcludingThis(&this->mTable, s_SizeOfStub,
+                                              mallocSizeOf, &args);
+    }
+    return PL_DHashTableSizeOfExcludingThis(&this->mTable, NULL, mallocSizeOf);
   }
 
 protected:
diff --git a/xpcom/glue/nsTHashtable.h b/xpcom/glue/nsTHashtable.h
index 762517fb0aa..e2a00135858 100644
--- a/xpcom/glue/nsTHashtable.h
+++ b/xpcom/glue/nsTHashtable.h
@@ -288,11 +288,14 @@ public:
   size_t SizeOfExcludingThis(SizeOfEntryExcludingThisFun sizeOfEntryExcludingThis,
                              nsMallocSizeOfFun mallocSizeOf, void *userArg = NULL) const
   {
-    if (IsInitialized()) {
+    if (!IsInitialized()) {
+      return 0;
+    }
+    if (sizeOfEntryExcludingThis) {
       s_SizeOfArgs args = { sizeOfEntryExcludingThis, userArg };
       return PL_DHashTableSizeOfExcludingThis(&mTable, s_SizeOfStub, mallocSizeOf, &args);
     }
-    return 0;
+    return PL_DHashTableSizeOfExcludingThis(&mTable, NULL, mallocSizeOf);
   }
 
 #ifdef DEBUG