diff --git a/mod/lti/OAuth.php b/mod/lti/OAuth.php
index 762d236ae72..3147692dd06 100644
--- a/mod/lti/OAuth.php
+++ b/mod/lti/OAuth.php
@@ -265,6 +265,11 @@ class OAuthSignatureMethod_RSA_SHA1 extends OAuthSignatureMethod {
         // Sign using the key
         $ok = openssl_sign($base_string, $signature, $privatekeyid);
 
+        // Avoid passing null values to base64_encode.
+        if (!$ok) {
+            throw new OAuthException("OpenSSL unable to sign data");
+        }
+
         // TODO: Remove this block once PHP 8.0 becomes required.
         if (PHP_MAJOR_VERSION < 8) {
             // Release the key resource
diff --git a/mod/lti/classes/local/ltiopenid/jwks_helper.php b/mod/lti/classes/local/ltiopenid/jwks_helper.php
index 8c9ed6217f3..56cb474d7a4 100644
--- a/mod/lti/classes/local/ltiopenid/jwks_helper.php
+++ b/mod/lti/classes/local/ltiopenid/jwks_helper.php
@@ -73,6 +73,11 @@ class jwks_helper {
         $res = openssl_pkey_get_private($privatekey['key']);
         $details = openssl_pkey_get_details($res);
 
+        // Avoid passing null values to base64_encode.
+        if (!isset($details['rsa']['e']) || !isset($details['rsa']['n'])) {
+            throw new \moodle_exception('Error: essential openssl keys not set');
+        }
+
         $jwk = array();
         $jwk['kty'] = 'RSA';
         $jwk['alg'] = 'RS256';