diff --git a/xpcom/string/nsReadableUtils.cpp b/xpcom/string/nsReadableUtils.cpp
index 2e14dab9c7d7..6576e6e5ebcc 100644
--- a/xpcom/string/nsReadableUtils.cpp
+++ b/xpcom/string/nsReadableUtils.cpp
@@ -1278,7 +1278,7 @@ EmptyCString()
 const nsString&
 NullString()
 {
-  static const nsXPIDLString sNull;
+  static const nsString sNull(mozilla::detail::StringDataFlags::VOIDED);
 
   return sNull;
 }
diff --git a/xpcom/string/nsTString.h b/xpcom/string/nsTString.h
index 5bd18a4d5c8f..027bf8797bcb 100644
--- a/xpcom/string/nsTString.h
+++ b/xpcom/string/nsTString.h
@@ -462,6 +462,15 @@ protected:
   {
   }
 
+  friend const nsTString_CharT& TNullString_CharT();
+
+  // Used by Null[C]String.
+  explicit nsTString_CharT(DataFlags aDataFlags)
+    : substring_type(char_traits::sEmptyBuffer, 0,
+                     aDataFlags | DataFlags::TERMINATED,
+                     ClassFlags::NULL_TERMINATED)
+  {}
+
   struct Segment {
     uint32_t mBegin, mLength;
     Segment(uint32_t aBegin, uint32_t aLength)
diff --git a/xpcom/string/string-template-def-char.h b/xpcom/string/string-template-def-char.h
index f8a38ad5bfd3..614b1addec60 100644
--- a/xpcom/string/string-template-def-char.h
+++ b/xpcom/string/string-template-def-char.h
@@ -25,3 +25,4 @@
 #define nsTPromiseFlatString_CharT          nsPromiseFlatCString
 #define TPromiseFlatString_CharT            PromiseFlatCString
 #define nsTSubstringSplitter_CharT          nsCSubstringSplitter
+#define TNullString_CharT                   NullCString
diff --git a/xpcom/string/string-template-def-unichar.h b/xpcom/string/string-template-def-unichar.h
index 2524b3b3f829..213a92ade919 100644
--- a/xpcom/string/string-template-def-unichar.h
+++ b/xpcom/string/string-template-def-unichar.h
@@ -25,3 +25,4 @@
 #define nsTPromiseFlatString_CharT          nsPromiseFlatString
 #define TPromiseFlatString_CharT            PromiseFlatString
 #define nsTSubstringSplitter_CharT          nsSubstringSplitter
+#define TNullString_CharT                   NullString
diff --git a/xpcom/string/string-template-undef.h b/xpcom/string/string-template-undef.h
index f3c31e29c3f5..c277b1ec72b4 100644
--- a/xpcom/string/string-template-undef.h
+++ b/xpcom/string/string-template-undef.h
@@ -26,3 +26,4 @@
 #undef nsTPromiseFlatString_CharT
 #undef TPromiseFlatString_CharT
 #undef nsTSubstringSplitter_CharT
+#undef TNullString_CharT