diff --git a/phpseclib/Net/SSH2.php b/phpseclib/Net/SSH2.php
index fe562d99..90c81d0d 100644
--- a/phpseclib/Net/SSH2.php
+++ b/phpseclib/Net/SSH2.php
@@ -2941,7 +2941,7 @@ class Net_SSH2 {
         $max_size = min(
             $this->packet_size_client_to_server[$client_channel],
             $this->window_size_client_to_server[$client_channel]
-        );
+        ) - 4;
         while (strlen($data) > $max_size) {
             $packet = pack('CN2a*',
                 NET_SSH2_MSG_CHANNEL_DATA,
@@ -2956,7 +2956,7 @@ class Net_SSH2 {
                 return false;
             }
 
-            if ($max_size == $this->window_size_client_to_server[$client_channel]) {
+            if ($max_size == $this->window_size_client_to_server[$client_channel] - 4) {
                 $this->bitmap^= NET_SSH2_MASK_WINDOW_ADJUST;
                 // using an invalid channel will let the buffers be built up for the valid channels
                 $this->_get_channel_packet(-1);
@@ -2964,11 +2964,11 @@ class Net_SSH2 {
                 $max_size = min(
                     $this->packet_size_client_to_server[$client_channel],
                     $this->window_size_client_to_server[$client_channel]
-                );
+                ) - 4;
             }
         }
 
-        if (strlen($data) >= $this->window_size_client_to_server[$client_channel]) {
+        if (strlen($data) >= $this->window_size_client_to_server[$client_channel] - 4) {
             $this->bitmap^= NET_SSH2_MASK_WINDOW_ADJUST;
             $this->_get_channel_packet(-1);
             $this->bitmap^= NET_SSH2_MASK_WINDOW_ADJUST;