diff --git a/category_handler.php b/category_handler.php
index bbe6ad555..9ca8bc728 100644
--- a/category_handler.php
+++ b/category_handler.php
@@ -62,7 +62,7 @@ function updateIconBlob($catId, $iconData, $iconMimeType) {
   if (!dbi_execute(
     'DELETE FROM webcal_categories
     WHERE cat_id = ? AND ( cat_owner = ?'
-      . ($is_admin ? ' OR cat_owner IS NULL )' : ' )'),
+      . ($is_admin ? ' OR cat_owner = \'\' )' : ' )'),
     [$id, $login]
   )) {
     $error = db_error();
@@ -95,16 +95,18 @@ function updateIconBlob($catId, $iconData, $iconMimeType) {
       $row = dbi_fetch_row($res);
       $id = $row[0] + 1;
       dbi_free_result($res);
-      // Set catowner to NULL for global category
-      $catowner = ($is_admin ? ($isglobal == 'Y' ? null : $login) : $login);
+      // Set catowner to empty string for global category
+      $catowner = ($is_admin ? ($isglobal == 'Y' ? '' : $login) : $login);
       if (!dbi_execute(
         'INSERT INTO webcal_categories ( cat_id, cat_owner,
         cat_name, cat_color ) VALUES ( ?, ?, ?, ? )',
         [$id, $catowner, $catname, $catcolor]
-      ))
+      )) {
         $error = db_error();
-    } else
+      }
+    } else {
       $error = db_error();
+    }
   }
   if (empty($delIcon) && (!empty($ENABLE_ICON_UPLOADS) && $ENABLE_ICON_UPLOADS == 'Y' || $is_admin)) {
     // Save icon if uploaded.
diff --git a/edit_entry.php b/edit_entry.php
index 63761b6b4..8677ac6b5 100644
--- a/edit_entry.php
+++ b/edit_entry.php
@@ -368,7 +368,7 @@ function time_selection($prefix, $time = '', $trigger = false)
   if (!empty($cat_id)) {
     $res = dbi_execute(
       'SELECT cat_name FROM webcal_categories ' .
-        'WHERE cat_id = ? AND ( cat_owner = ? OR cat_owner IS NULL )',
+        'WHERE cat_id = ? AND ( cat_owner = ? OR cat_owner = \'\' )',
       [$cat_id, $real_user]
     );
     if ($res) {
diff --git a/includes/functions.php b/includes/functions.php
index 30e63678f..82eaf8c03 100644
--- a/includes/functions.php
+++ b/includes/functions.php
@@ -2471,7 +2471,7 @@ function get_categories_by_id ( $id, $user, $asterisk = false ) {
 
   $res = dbi_execute ( 'SELECT wc.cat_name, wc.cat_id, wec.cat_owner
     FROM webcal_categories wc, webcal_entry_categories wec WHERE wec.cal_id = ?
-    AND wec.cat_id = wc.cat_id AND ( wc.cat_owner = ? OR wc.cat_owner IS NULL )
+    AND wec.cat_id = wc.cat_id AND ( wc.cat_owner = ? OR wc.cat_owner = \'\' )
   ORDER BY wec.cat_order', [$id, ( empty ( $user ) ? $login : $user )] );
   while ( $row = dbi_fetch_row ( $res ) ) {
     $categories[ ( empty ( $row[2] ) ? - $row[1] : $row[1] ) ] = $row[0]
@@ -4209,7 +4209,7 @@ function load_user_categories ( $ex_global = '' ) {
       ( $is_assistant || $is_admin ) ? $user : $login );
     $rows = dbi_get_cached_rows ( 'SELECT cat_id, cat_name, cat_owner, cat_color, cat_icon_mime
       FROM webcal_categories WHERE ( cat_owner = ? ) ' . ( $ex_global == ''
-        ? 'OR ( cat_owner IS NULL ) ORDER BY cat_owner,' : 'ORDER BY' )
+        ? 'OR ( cat_owner = \'\' ) ORDER BY cat_owner,' : 'ORDER BY' )
        . ' cat_name', $query_params );
     if ( $rows ) {
       for ( $i = 0, $cnt = count ( $rows ); $i < $cnt; $i++ ) {
diff --git a/includes/xcal.php b/includes/xcal.php
index 7ac151f91..e423785c9 100644
--- a/includes/xcal.php
+++ b/includes/xcal.php
@@ -3121,7 +3121,7 @@ function get_categories_id_byname ( $cat_names ) {
   $categories = explode ( ',', $cat_names );
   foreach ( $categories as $cat_name ) {
     $res = dbi_execute ( 'SELECT cat_id FROM webcal_categories
-      WHERE cat_name  = ? AND ( cat_owner = ? OR cat_owner IS NULL )',
+      WHERE cat_name  = ? AND ( cat_owner = ? OR cat_owner = \'\' )',
         [$cat_name, $login] );
     if ( $res ) {
       if ( $row = dbi_fetch_row ( $res ) ) {
diff --git a/install/sql/tables-mysql.sql b/install/sql/tables-mysql.sql
index c0bb7ebd1..8d370c0e6 100644
--- a/install/sql/tables-mysql.sql
+++ b/install/sql/tables-mysql.sql
@@ -414,7 +414,7 @@ CREATE TABLE webcal_entry_log (
 
 /**
  * Defines user categories.  Categories can be specific to a user or global.
- * When a category is global, the cat_owner field will be NULL.
+ * When a category is global, the cat_owner field will be an empty string.
  * (Only an admin user can create a global category.)
  */
 CREATE TABLE webcal_categories (