diff --git a/mozilla/string/src/nsXPIDLString.cpp b/mozilla/string/src/nsXPIDLString.cpp
index 5ea9266b732..f3c7b9c3422 100644
--- a/mozilla/string/src/nsXPIDLString.cpp
+++ b/mozilla/string/src/nsXPIDLString.cpp
@@ -156,7 +156,7 @@ nsXPIDLString::GetSharedEmptyBufferHandle()
                                          // race to set the refcount, in
                                          // which case we'll pull the
                                          // same trick in |Destroy|.
-      sBufferHandle->SetImplementationFlags(sBufferHandle->GetImplementationFlags() & shared_buffer_handle_type::kIsNULL);
+      sBufferHandle->SetImplementationFlags(sBufferHandle->GetImplementationFlags() | shared_buffer_handle_type::kIsNULL);
     }
     return sBufferHandle;
   }
@@ -239,7 +239,7 @@ nsXPIDLCString::GetSharedEmptyBufferHandle()
                                          // race to set the refcount, in
                                          // which case we'll pull the
                                          // same trick in |Destroy|.
-      sBufferHandle->SetImplementationFlags(sBufferHandle->GetImplementationFlags() & shared_buffer_handle_type::kIsNULL);
+      sBufferHandle->SetImplementationFlags(sBufferHandle->GetImplementationFlags() | shared_buffer_handle_type::kIsNULL);
     }
     return sBufferHandle;
   }
diff --git a/mozilla/xpcom/string/src/nsXPIDLString.cpp b/mozilla/xpcom/string/src/nsXPIDLString.cpp
index 5ea9266b732..f3c7b9c3422 100644
--- a/mozilla/xpcom/string/src/nsXPIDLString.cpp
+++ b/mozilla/xpcom/string/src/nsXPIDLString.cpp
@@ -156,7 +156,7 @@ nsXPIDLString::GetSharedEmptyBufferHandle()
                                          // race to set the refcount, in
                                          // which case we'll pull the
                                          // same trick in |Destroy|.
-      sBufferHandle->SetImplementationFlags(sBufferHandle->GetImplementationFlags() & shared_buffer_handle_type::kIsNULL);
+      sBufferHandle->SetImplementationFlags(sBufferHandle->GetImplementationFlags() | shared_buffer_handle_type::kIsNULL);
     }
     return sBufferHandle;
   }
@@ -239,7 +239,7 @@ nsXPIDLCString::GetSharedEmptyBufferHandle()
                                          // race to set the refcount, in
                                          // which case we'll pull the
                                          // same trick in |Destroy|.
-      sBufferHandle->SetImplementationFlags(sBufferHandle->GetImplementationFlags() & shared_buffer_handle_type::kIsNULL);
+      sBufferHandle->SetImplementationFlags(sBufferHandle->GetImplementationFlags() | shared_buffer_handle_type::kIsNULL);
     }
     return sBufferHandle;
   }