diff --git a/lib/externallib.php b/lib/externallib.php
index 8f20b35456b..7fd7e52abe8 100644
--- a/lib/externallib.php
+++ b/lib/externallib.php
@@ -48,7 +48,7 @@ function external_function_info($function, $strictness=MUST_EXIST) {
         // Fallback to explicit include of externallib.php.
         $function->classpath = empty($function->classpath) ? core_component::get_component_directory($function->component).'/externallib.php' : $CFG->dirroot.'/'.$function->classpath;
         if (!file_exists($function->classpath)) {
-            throw new coding_exception('Cannot find file with external function implementation');
+            throw new coding_exception('Cannot find file with external function implementation: ' . $function->classname);
         }
         require_once($function->classpath);
         if (!class_exists($function->classname)) {
@@ -551,6 +551,30 @@ class external_multiple_structure extends external_description {
  * @since Moodle 2.0
  */
 class external_function_parameters extends external_single_structure {
+
+    /**
+     * Constructor - does extra checking to prevent top level optional parameters.
+     *
+     * @param array $keys
+     * @param string $desc
+     * @param bool $required
+     * @param array $default
+     */
+    public function __construct(array $keys, $desc='', $required=VALUE_REQUIRED, $default=null) {
+        global $CFG;
+
+        if ($CFG->debugdeveloper) {
+            foreach ($keys as $key => $value) {
+                if ($value instanceof external_value) {
+                    if ($value->required == VALUE_OPTIONAL) {
+                        debugging('External function parameters: invalid OPTIONAL value specified.', DEBUG_DEVELOPER);
+                        break;
+                    }
+                }
+            }
+        }
+        parent::__construct($keys, $desc, $required, $default);
+    }
 }
 
 /**
diff --git a/lib/tests/externallib_test.php b/lib/tests/externallib_test.php
index 0caa69eb8bf..63d7b519165 100644
--- a/lib/tests/externallib_test.php
+++ b/lib/tests/externallib_test.php
@@ -264,6 +264,26 @@ class core_externallib_testcase extends advanced_testcase {
         $this->setExpectedException('invalid_parameter_exception');
         test_exernal_api::get_context_wrapper(array('roleid' => 3, 'userid' => $USER->id, 'instanceid' => $course->id));
     }
+
+    public function test_all_external_info() {
+        global $DB;
+
+        $functions = $DB->get_records('external_functions', array(), 'name');
+        // We are testing here that all the external function descriptions can be generated without
+        // producing warnings. E.g. misusing optional params will generate a debugging message which
+        // will fail this test.
+        foreach ($functions as $f) {
+            $desc = external_function_info($f);
+            $this->assertNotEmpty($desc->name);
+            $this->assertNotEmpty($desc->classname);
+            $this->assertNotEmpty($desc->methodname);
+            $this->assertEquals($desc->component, clean_param($desc->component, PARAM_COMPONENT));
+            $this->assertInstanceOf('external_function_parameters', $desc->parameters_desc);
+            if ($desc->returns_desc != null) {
+                $this->assertInstanceOf('external_description', $desc->returns_desc);
+            }
+        }
+    }
 }
 
 /*