diff --git a/core/includes/form.inc b/core/includes/form.inc
index 06ee164..638d922 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -131,10 +131,14 @@
       $form_state['build_info']['base_form_id'] = $form_arg->getBaseFormID();
     }
     return $form_arg->getFormID();
-  }
-
-  // Otherwise, the $form_arg is the form ID.
-  return $form_arg;
+  } else if (function_exists($form_arg)) {
+    // Otherwise, the $form_arg is the form ID.
+    return $form_arg;
+  } else {
+    // The form does not exist or the file with it's definition hasn't been
+    // loaded yet.
+    return FALSE;
+   }
 }
 
 /**
@@ -174,7 +178,9 @@
   $form_state['build_info']['args'] = $args;
 
   $form_id = _drupal_form_id($form_arg, $form_state);
-  return drupal_build_form($form_id, $form_state);
+  // If the definition of the form does not exist($form_id = FALSE),
+  // we return an empty array so nothing is rendered.
+  return $form_id ? drupal_build_form($form_id, $form_state) : array();
 }
 
 /**