diff --git a/library/chacha20.c b/library/chacha20.c
index b20c7ad..3511245 100644
--- a/library/chacha20.c
+++ b/library/chacha20.c
@@ -291,10 +291,15 @@
     size_t offset = 0U;
     size_t i;
 
-    if ( ( ctx == NULL ) || ( input == NULL ) || ( output == NULL ) )
+    if ( ctx == NULL )
     {
         return( MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA );
     }
+    else if ( ( size > 0U ) && ( ( input == NULL ) || ( output == NULL ) ) )
+    {
+        /* input and output pointers are allowed to be NULL only if size == 0 */
+        return( MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA );
+    }
 
     /* Use leftover keystream bytes, if available */
     while ( ( size > 0U ) && ( ctx->keystream_bytes_used < CHACHA20_BLOCK_SIZE_BYTES ) )