diff --git a/libcxx/src/support/win32/support.cpp b/libcxx/src/support/win32/support.cpp
index b4f1e22f3c61..4c9a4b3a507e 100644
--- a/libcxx/src/support/win32/support.cpp
+++ b/libcxx/src/support/win32/support.cpp
@@ -61,6 +61,11 @@ size_t mbsnrtowcs( wchar_t *__restrict dst, const char **__restrict src,
     size_t result = 0;
     bool have_result = false;
 
+    // If dst is null then max_dest_chars should be ignored according to the
+    // standard.  Setting max_dest_chars to a large value has this effect.
+    if (!dst)
+        max_dest_chars = static_cast<size_t>(-1);
+
     while ( source_remaining ) {
         if ( dst && dest_converted >= max_dest_chars )
             break;
@@ -114,6 +119,11 @@ size_t wcsnrtombs( char *__restrict dst, const wchar_t **__restrict src,
     bool have_result = false;
     bool terminator_found = false;
 
+    // If dst is null then dst_size_bytes should be ignored according to the
+    // standard.  Setting dest_remaining to a large value has this effect.
+    if (!dst)
+        dest_remaining = static_cast<size_t>(-1);
+
     while ( source_converted != max_source_chars ) {
         if ( ! dest_remaining )
             break;