diff --git a/xs/src/libslic3r/GCodeSender.cpp b/xs/src/libslic3r/GCodeSender.cpp
index bb91a84c1a..c880ee4089 100644
--- a/xs/src/libslic3r/GCodeSender.cpp
+++ b/xs/src/libslic3r/GCodeSender.cpp
@@ -355,6 +355,7 @@ GCodeSender::on_read(const boost::system::error_code& error,
             // extract the first number from line
             boost::algorithm::trim_left_if(line, !boost::algorithm::is_digit());
             size_t toresend = boost::lexical_cast<size_t>(line.substr(0, line.find_first_not_of("0123456789")));
+            toresend++; // N is 0-based
             if (toresend >= this->sent - this->last_sent.size()) {
                 {
                     boost::lock_guard<boost::mutex> l(this->queue_mutex);
@@ -473,8 +474,8 @@ GCodeSender::do_send()
     if (line.empty()) return;
     
     // compute full line
-    this->sent++;
     std::string full_line = "N" + boost::lexical_cast<std::string>(this->sent) + " " + line;
+    this->sent++;
     
     // calculate checksum
     int cs = 0;