diff --git a/library/rsa.c b/library/rsa.c
index b680188..25544aa 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -1624,9 +1624,15 @@
                       plaintext_max_size,
                       plaintext_max_size - plaintext_size );
 
-    /* Finally copy the decrypted plaintext plus trailing zeros
-     * into the output buffer. */
-    memcpy( output, buf + ilen - plaintext_max_size, plaintext_max_size );
+    /* Finally copy the decrypted plaintext plus trailing zeros into the output
+     * buffer. If output_max_len is 0, then output may be an invalid pointer
+     * and the result of memcpy() would be undefined; prevent undefined
+     * behavior making sure to depend only on output_max_len (the size of the
+     * user-provided output buffer), which is independent from plaintext
+     * length, validity of padding, success of the decryption, and other
+     * secrets. */
+    if( output_max_len != 0 )
+        memcpy( output, buf + ilen - plaintext_max_size, plaintext_max_size );
 
     /* Report the amount of data we copied to the output buffer. In case
      * of errors (bad padding or output too large), the value of *olen