diff --git a/auth/cas/auth.php b/auth/cas/auth.php
index 56e5a7de9fd..c29c9205e81 100644
--- a/auth/cas/auth.php
+++ b/auth/cas/auth.php
@@ -365,13 +365,17 @@ class auth_plugin_cas extends auth_plugin_ldap {
             return [];
         }
 
-        $iconurl = moodle_url::make_pluginfile_url(
-            context_system::instance()->id,
-            'auth_cas',
-            'logo',
-            null,
-            '/',
-            $this->config->auth_logo);
+        if ($this->config->auth_logo) {
+            $iconurl = moodle_url::make_pluginfile_url(
+                context_system::instance()->id,
+                'auth_cas',
+                'logo',
+                null,
+                null,
+                $this->config->auth_logo);
+        } else {
+            $iconurl = null;
+        }
 
         return [
             [
diff --git a/auth/shibboleth/auth.php b/auth/shibboleth/auth.php
index c52ad129135..ad0c55fbdee 100644
--- a/auth/shibboleth/auth.php
+++ b/auth/shibboleth/auth.php
@@ -294,12 +294,19 @@ class auth_plugin_shibboleth extends auth_plugin_base {
         }
 
         $url = new moodle_url('/auth/shibboleth/index.php');
-        $iconurl = moodle_url::make_pluginfile_url(context_system::instance()->id,
-                                                   'auth_shibboleth',
-                                                   'logo',
-                                                   null,
-                                                   '/',
-                                                   $config->auth_logo);
+
+        if ($config->auth_logo) {
+            $iconurl = moodle_url::make_pluginfile_url(
+                context_system::instance()->id,
+                'auth_shibboleth',
+                'logo',
+                null,
+                null,
+                $config->auth_logo);
+        } else {
+            $iconurl = null;
+        }
+
         $result[] = ['url' => $url, 'iconurl' => $iconurl, 'name' => $config->login_name];
         return $result;
     }