diff --git a/a8/websocketsession.cc b/a8/websocketsession.cc
index 0bd4eb8..44303c7 100644
--- a/a8/websocketsession.cc
+++ b/a8/websocketsession.cc
@@ -254,7 +254,7 @@ namespace a8
             unsigned char *frame_mask = (unsigned char*)(real_buf + mask_offset);
             memcpy(&decoded_buff_[decoded_bufflen_], real_buf + mask_offset + 4, framelen);
             for (unsigned int i = 0; i < framelen; i++) {
-                decoded_buff_[decoded_bufflen_ + i] = (decoded_buff_[i] ^ frame_mask[i%4]);
+                decoded_buff_[decoded_bufflen_ + i] = (decoded_buff_[decoded_bufflen_ + i] ^ frame_mask[i%4]);
             }
         } else {
             memcpy(&decoded_buff_[decoded_bufflen_], real_buf + mask_offset, framelen);