diff --git a/libc/stdio/lib_vsnprintf.c b/libc/stdio/lib_vsnprintf.c
index d25fc0d2d5..40f7dffc50 100644
--- a/libc/stdio/lib_vsnprintf.c
+++ b/libc/stdio/lib_vsnprintf.c
@@ -55,17 +55,38 @@
 int vsnprintf(FAR char *buf, size_t size, FAR const IPTR char *format,
               va_list ap)
 {
-  struct lib_memoutstream_s memoutstream;
-  int     n;
+  union
+  {
+    struct lib_outstream_s    nulloutstream;
+    struct lib_memoutstream_s memoutstream;
+  } u;
 
-  /* Initialize a memory stream to write to the buffer */
+  FAR struct lib_outstream_s *stream;
+  int n;
 
-  lib_memoutstream((FAR struct lib_memoutstream_s *)&memoutstream,
-                    buf, size);
+  /* "If the value of [size] is zero on a call to vsnprintf(), nothing shall
+   *  be written, the number of bytes that would have been written had [size]
+   *  been sufficiently large excluding the terminating null shall be returned,
+   *  and [buf] may be a null pointer." -- opengroup.org
+   */
+
+  if (size > 0)
+    {
+      /* Initialize a memory stream to write to the buffer */
+
+      lib_memoutstream(&u.memoutstream, buf, size);
+      stream = &u.memoutstream.public;
+    }
+  else
+    {
+      /* Use a null stream to get the size of the buffer */
+
+      lib_nulloutstream(&u.nulloutstream);
+      stream = &u.nulloutstream;
+    }
 
   /* Then let lib_vsprintf do the real work */
 
-  n = lib_vsprintf((FAR struct lib_outstream_s *)&memoutstream.public,
-                   format, ap);
+  n = lib_vsprintf(stream, format, ap);
   return n;
 }