diff --git a/libraries/GCS_MAVLink/GCS_Common.cpp b/libraries/GCS_MAVLink/GCS_Common.cpp
index feae99abd7..cf5680ae01 100644
--- a/libraries/GCS_MAVLink/GCS_Common.cpp
+++ b/libraries/GCS_MAVLink/GCS_Common.cpp
@@ -6636,14 +6636,14 @@ void GCS::passthru_timer(void)
     uint8_t buf[64];
 
     // read from port1, and write to port2
-    int16_t nbytes = _passthru.port1->read_locked(buf, sizeof(buf), lock_key);
+    int16_t nbytes = _passthru.port1->read_locked(buf, MIN(sizeof(buf),_passthru.port2->txspace()), lock_key);
     if (nbytes > 0) {
         _passthru.last_port1_data_ms = AP_HAL::millis();
         _passthru.port2->write_locked(buf, nbytes, lock_key);
     }
 
     // read from port2, and write to port1
-    nbytes = _passthru.port2->read_locked(buf, sizeof(buf), lock_key);
+    nbytes = _passthru.port2->read_locked(buf, MIN(sizeof(buf),_passthru.port1->txspace()), lock_key);
     if (nbytes > 0) {
         _passthru.port1->write_locked(buf, nbytes, lock_key);
     }