diff --git a/OpenSSL/SSL.py b/OpenSSL/SSL.py
index dc983b5..675e8a6 100644
--- a/OpenSSL/SSL.py
+++ b/OpenSSL/SSL.py
@@ -975,8 +975,8 @@
         # Build a C string from the list. We don't need to save this off
         # because OpenSSL immediately copies the data out.
         input_str = _ffi.new("unsigned char[]", protostr)
-        input_str_len = _ffi.new("unsigned", len(protostr))
-        _lib.SSL_CTX_set_alpn_protos(self._context, input_str)
+        input_str_len = _ffi.cast("unsigned", len(protostr))
+        _lib.SSL_CTX_set_alpn_protos(self._context, input_str, input_str_len)
 
     def set_alpn_select_callback(self, callback):
         """