diff --git a/lang/en_utf8/webservice.php b/lang/en_utf8/webservice.php
index 9948c719e40..02f33d9424b 100644
--- a/lang/en_utf8/webservice.php
+++ b/lang/en_utf8/webservice.php
@@ -59,6 +59,7 @@ $string['functions'] = 'Functions';
 $string['generalstructure'] = 'General structure';
 $string['httpswarning'] = 'Token strings are only displayed if your connection is secured (https)';
 $string['information'] = 'Information';
+$string['invalidextparam'] = 'Invalid external api parameter: $a';
 $string['invalidiptoken'] = 'Invalid token - your IP is not supported';
 $string['invalidtimedtoken'] = 'Invalid token - token expired';
 $string['invalidtoken'] = 'Invalid token - token not found';
diff --git a/lib/externallib.php b/lib/externallib.php
index 65d72325fae..055a8cd2bc2 100644
--- a/lib/externallib.php
+++ b/lib/externallib.php
@@ -168,11 +168,19 @@ class external_api {
                     }
                     if ($subdesc instanceof external_value) {
                             if ($subdesc->required == VALUE_DEFAULT) {
-                                $result[$key] = self::validate_parameters($subdesc, $subdesc->default);
+                                try {
+                                    $result[$key] = self::validate_parameters($subdesc, $subdesc->default);
+                                } catch (invalid_parameter_exception $e) {
+                                    throw new webservice_parameter_exception('invalidextparam',$key);
+                                }
                             }
                         }
                 } else {
-                    $result[$key] = self::validate_parameters($subdesc, $params[$key]);
+                    try {
+                        $result[$key] = self::validate_parameters($subdesc, $params[$key]);
+                    } catch (invalid_parameter_exception $e) {
+                        throw new webservice_parameter_exception('invalidextparam',$key);
+                    }
                 }
                 unset($params[$key]);
             }
diff --git a/lib/setuplib.php b/lib/setuplib.php
index 2aa157b3f19..8170784180d 100644
--- a/lib/setuplib.php
+++ b/lib/setuplib.php
@@ -91,6 +91,23 @@ class moodle_exception extends Exception {
     }
 }
 
+/**
+ * Web service parameter exception class
+ *
+ * This exception must be thrown to the web service client when a web service parameter is invalid
+ * The error string is gotten from webservice.php
+ */
+class webservice_parameter_exception extends moodle_exception {
+    /**
+     * Constructor
+     * @param string $errorcode The name of the string from webservice.php to print
+     * @param string $a The name of the parameter
+     */
+    function __construct($errorcode=null, $a = '') {
+        parent::__construct($errorcode, 'webservice', '', $a, null);
+    }
+}
+
 /**
  * Exceptions indicating user does not have permissions to do something
  * and the execution can not continue.