diff --git a/lib/outputlib.php b/lib/outputlib.php
index 64e02997da4..7dd382ba465 100644
--- a/lib/outputlib.php
+++ b/lib/outputlib.php
@@ -447,10 +447,15 @@ class theme_config {
             throw new coding_exception('Default theme '.theme_config::DEFAULT_THEME.' not available or broken!');
 
         } else if ($config = theme_config::find_theme_config($CFG->theme, $settings)) {
+            debugging('This page should be using theme ' . $themename .
+                    ' which cannot be initialised. Falling back to the site theme ' . $CFG->theme, DEBUG_NORMAL);
             return new theme_config($config);
 
         } else {
             // bad luck, the requested theme has some problems - admin see details in theme config
+            debugging('This page should be using theme ' . $themename .
+                    ' which cannot be initialised. Nor can the site theme ' . $CFG->theme .
+                    '. Falling back to ' . theme_config::DEFAULT_THEME, DEBUG_NORMAL);
             return new theme_config(theme_config::find_theme_config(theme_config::DEFAULT_THEME, $settings));
         }
     }