diff --git a/calendar/lib.php b/calendar/lib.php
index dc56f91fd63..8200b980601 100644
--- a/calendar/lib.php
+++ b/calendar/lib.php
@@ -2866,8 +2866,7 @@ function calendar_process_subscription_row($subscriptionid, $pollinterval, $acti
             return "<p>".get_string('subscriptionupdated', 'calendar', $sub->name)."</p>" . calendar_update_subscription_events($subscriptionid);
 
         case $strremove:
-            $DB->delete_records('event', array('subscriptionid' => $subscriptionid));
-            $DB->delete_records('event_subscriptions', array('id' => $subscriptionid));
+            calendar_delete_subscription($subscriptionid);
             return get_string('subscriptionremoved', 'calendar', $sub->name);
             break;
 
@@ -2877,6 +2876,21 @@ function calendar_process_subscription_row($subscriptionid, $pollinterval, $acti
     return '';
 }
 
+/**
+ * Delete subscription and all related events.
+ *
+ * @param int|stdClass $subscription subscription or it's id, which needs to be deleted.
+ */
+function calendar_delete_subscription($subscription) {
+    global $DB;
+
+    if (is_object($subscription)) {
+        $subscription = $subscription->id;
+    }
+    // Delete subscription and related events.
+    $DB->delete_records('event', array('subscriptionid' => $subscription));
+    $DB->delete_records('event_subscriptions', array('id' => $subscription));
+}
 /**
  * From a URL, fetch the calendar and return an iCalendar object.
  *
diff --git a/calendar/managesubscriptions.php b/calendar/managesubscriptions.php
index cc726f13d70..07265bc9213 100644
--- a/calendar/managesubscriptions.php
+++ b/calendar/managesubscriptions.php
@@ -75,7 +75,13 @@ if (!empty($formdata)) {
         $ical->unserialize($calendar);
         $importresults = calendar_import_icalendar_events($ical, $courseid, $subscriptionid);
     } else {
-        $importresults = calendar_update_subscription_events($subscriptionid);
+        try {
+            $importresults = calendar_update_subscription_events($subscriptionid);
+        } catch (moodle_exception $e) {
+            // Delete newly added subscription and show invalid url error.
+            calendar_delete_subscription($subscriptionid);
+            print_error($e->errorcode, $e->module, $PAGE->url);
+        }
     }
     // Redirect to prevent refresh issues.
     redirect($PAGE->url, $importresults);
@@ -83,7 +89,12 @@ if (!empty($formdata)) {
     // The user is wanting to perform an action upon an existing subscription.
     require_sesskey(); // Must have sesskey for all actions.
     if (calendar_can_edit_subscription($subscriptionid)) {
-        $importresults = calendar_process_subscription_row($subscriptionid, $pollinterval, $action);
+        try {
+            $importresults = calendar_process_subscription_row($subscriptionid, $pollinterval, $action);
+        } catch (moodle_exception $e) {
+            // If exception caught, then user should be redirected to page where he/she came from.
+            print_error($e->errorcode, $e->module, $PAGE->url);
+        }
     } else {
         print_error('nopermissions', 'error', $PAGE->url, get_string('managesubscriptions', 'calendar'));
     }
diff --git a/calendar/managesubscriptions_form.php b/calendar/managesubscriptions_form.php
index 28344cd64ec..a969fbd4595 100644
--- a/calendar/managesubscriptions_form.php
+++ b/calendar/managesubscriptions_form.php
@@ -123,7 +123,9 @@ class calendar_addsubscription_form extends moodleform {
                 }
             }
         } else if (($data['importfrom'] == CALENDAR_IMPORT_FROM_URL)) {
-            if (clean_param($data['url'], PARAM_URL) !== $data['url']) {
+            // Clean input calendar url.
+            $url = clean_param($data['url'], PARAM_URL);
+            if (empty($url) || ($url !== $data['url'])) {
                 $errors['url']  = get_string('invalidurl', 'error');
             }
         } else {