diff --git a/libcxx/include/fstream b/libcxx/include/fstream
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -754,7 +754,8 @@
         else
         {
             if (__extbufend_ != __extbufnext_) {
-                _LIBCPP_ASSERT(__extbufnext_ != nullptr, "underflow moving from NULL" );
+                _LIBCPP_ASSERT(__extbufnext_ != nullptr, "underflow moving from nullptr");
+                _LIBCPP_ASSERT(__extbuf_ != nullptr, "underflow moving into nullptr");
                 _VSTD::memmove(__extbuf_, __extbufnext_, __extbufend_ - __extbufnext_);
             }
             __extbufnext_ = __extbuf_ + (__extbufend_ - __extbufnext_);
diff --git a/libcxx/include/locale b/libcxx/include/locale
--- a/libcxx/include/locale
+++ b/libcxx/include/locale
@@ -4014,7 +4014,8 @@
         else
         {
              if (__extbufend_ != __extbufnext_) {
-                _LIBCPP_ASSERT(__extbufnext_ != NULL, "underflow moving from NULL" );
+                _LIBCPP_ASSERT(__extbufnext_ != nullptr, "underflow moving from nullptr");
+                _LIBCPP_ASSERT(__extbuf_ != nullptr, "underflow moving into nullptr");
                 _VSTD::memmove(__extbuf_, __extbufnext_, __extbufend_ - __extbufnext_);
              }
             __extbufnext_ = __extbuf_ + (__extbufend_ - __extbufnext_);