diff --git a/lang/en/error.php b/lang/en/error.php
index 4d7cde810cc..2faf5302ead 100644
--- a/lang/en/error.php
+++ b/lang/en/error.php
@@ -495,6 +495,8 @@ $string['onlyadmins'] = 'Only administrators can do that';
 $string['onlyeditingteachers'] = 'Only editing teachers can do that';
 $string['onlyeditown'] = 'You can only edit your own information';
 $string['orderidnotfound'] = 'Order ID {$a} not found';
+$string['opensslsignerror'] = 'OpenSSL unable to sign data';
+$string['opensslsealerror'] = 'OpenSSL unable to seal data';
 $string['pagenotexisttitle'] = '404 Error: File not found';
 $string['pagenotexist'] = '<p>An unusual error occurred trying to view a page that does not exist:</p>{$a}';
 $string['pathdoesnotstartslash'] = 'No valid arguments supplied, path does not start with slash!';
diff --git a/mnet/lib.php b/mnet/lib.php
index 20a14a59c36..3c615271cc7 100644
--- a/mnet/lib.php
+++ b/mnet/lib.php
@@ -216,7 +216,12 @@ function mnet_sign_message($message, $privatekey = null) {
     // The '$sig' value below is returned by reference.
     // We initialize it first to stop my IDE from complaining.
     $sig  = '';
-    $bool = openssl_sign($message, $sig, $privatekey); // TODO: On failure?
+    $bool = openssl_sign($message, $sig, $privatekey);
+
+    // Avoid passing null values to base64_encode.
+    if ($bool === false) {
+        throw new \moodle_exception('opensslsignerror');
+    }
 
     $message = '<?xml version="1.0" encoding="iso-8859-1"?>
     <signedMessage>
@@ -283,6 +288,12 @@ function mnet_encrypt_message($message, $remote_certificate) {
 
     //        passed by ref ->     &$encryptedstring &$symmetric_keys
     $bool = openssl_seal($message, $encryptedstring, $symmetric_keys, array($publickey), 'RC4');
+
+    // Avoid passing null values to base64_encode.
+    if ($bool === false) {
+        throw new \moodle_exception('opensslsealerror');
+    }
+
     $message = $encryptedstring;
     $symmetrickey = array_pop($symmetric_keys);