diff --git a/indefero/src/IDF/Form/Upload.php b/indefero/src/IDF/Form/Upload.php
index c49520e..f0018ee 100644
--- a/indefero/src/IDF/Form/Upload.php
+++ b/indefero/src/IDF/Form/Upload.php
@@ -146,11 +146,13 @@ class IDF_Form_Upload extends Pluf_Form
             throw new Pluf_Form_Invalid(__("Must upload a file or specify an external file"));
         }
 
-        foreach(explode(".", $this->cleaned_data["ext_file_name"]) as $section) {
-            if (!ctype_alnum(($section))) {
-                throw new Pluf_Form_Invalid(__("External file name must not contain characters other than a-zA-Z and ."));
-            }
-        }
+		if ($this->cleaned_data["ext_file_name"] != "") {
+			foreach(explode(".", $this->cleaned_data["ext_file_name"]) as $section) {
+				if (!ctype_alnum(($section))) {
+					throw new Pluf_Form_Invalid(__("External file name must not contain characters other than a-zA-Z and ."));
+				}
+			}
+		}
         return $this->cleaned_data;
     }