diff --git a/lib/externallib.php b/lib/externallib.php
index 002a99d2dee..50cd9da668d 100644
--- a/lib/externallib.php
+++ b/lib/externallib.php
@@ -84,11 +84,12 @@ class external_api {
                 $function->classpath = $CFG->dirroot.'/'.$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 ' . $function->classpath .
+                        ' with external function implementation');
             }
             require_once($function->classpath);
             if (!class_exists($function->classname)) {
-                throw new coding_exception('Cannot find external class');
+                throw new coding_exception('Cannot find external class ' . $function->classname);
             }
         }
 
@@ -99,13 +100,16 @@ class external_api {
 
         // Make sure the implementaion class is ok.
         if (!method_exists($function->classname, $function->methodname)) {
-            throw new coding_exception('Missing implementation method of '.$function->classname.'::'.$function->methodname);
+            throw new coding_exception('Missing implementation method ' .
+                    $function->classname . '::' . $function->methodname);
         }
         if (!method_exists($function->classname, $function->parameters_method)) {
-            throw new coding_exception('Missing parameters description');
+            throw new coding_exception('Missing parameters description method ' .
+                    $function->classname . '::' . $function->parameters_method);
         }
         if (!method_exists($function->classname, $function->returns_method)) {
-            throw new coding_exception('Missing returned values description');
+            throw new coding_exception('Missing returned values description method ' .
+                    $function->classname . '::' . $function->returns_method);
         }
         if (method_exists($function->classname, $function->deprecated_method)) {
             if (call_user_func(array($function->classname, $function->deprecated_method)) === true) {
@@ -117,14 +121,16 @@ class external_api {
         // Fetch the parameters description.
         $function->parameters_desc = call_user_func(array($function->classname, $function->parameters_method));
         if (!($function->parameters_desc instanceof external_function_parameters)) {
-            throw new coding_exception('Invalid parameters description');
+            throw new coding_exception($function->classname . '::' . $function->parameters_method .
+                    ' did not return a valid external_function_parameters object.');
         }
 
         // Fetch the return values description.
         $function->returns_desc = call_user_func(array($function->classname, $function->returns_method));
         // Null means void result or result is ignored.
         if (!is_null($function->returns_desc) and !($function->returns_desc instanceof external_description)) {
-            throw new coding_exception('Invalid return description');
+            throw new coding_exception($function->classname . '::' . $function->returns_method .
+                    ' did not return a valid external_description object');
         }
 
         // Now get the function description.