diff --git a/application/models/Logbook_model.php b/application/models/Logbook_model.php
index 5757b59a..ac73805d 100755
--- a/application/models/Logbook_model.php
+++ b/application/models/Logbook_model.php
@@ -175,8 +175,8 @@ class Logbook_model extends CI_Model {
             'COL_TX_PWR' => $tx_power,
             'COL_STX' => $stx,
             'COL_SRX' => $srx,
-            'COL_STX_STRING' => strtoupper(trim($stx_string)),
-            'COL_SRX_STRING' => strtoupper(trim($srx_string)),
+            'COL_STX_STRING' => $stx_string == null ? '' : strtoupper(trim($stx_string)),
+            'COL_SRX_STRING' => $srx_string == null ? '' : strtoupper(trim($srx_string)),
             'COL_CONTEST_ID' => $contestid,
             'COL_NR_BURSTS' => null,
             'COL_NR_PINGS' => null,