diff --git a/functions.http.inc.php b/functions.http.inc.php
index f718179..66ba15c 100644
--- a/functions.http.inc.php
+++ b/functions.http.inc.php
@@ -125,7 +125,13 @@ function getMandatoryPostParameter($parameterName, $message = '', $json = false)
 }
 
 function getGetParameter($parameterName) {
-    return $_GET[$parameterName];
+    if (array_key_exists($parameterName, $_GET)
+        && isset($_GET[$parameterName])
+        && !empty($_GET[$parameterName])) {
+        return $_GET[$parameterName];
+    } else {
+        return NULL;
+    }
 }
 
 function getOptionalGetParameter($parameterName, $default = NULL) {