diff --git a/config.php b/config.php
index b611108..52036c4 100644
--- a/config.php
+++ b/config.php
@@ -88,37 +88,34 @@ class config
     $rc = include ($this->confFile);
     if ($rc !== 1)
       throw new Exception ("Error in configuration file", 500);
-    if (array_key_exists ($param, $conf))
+    if (! array_key_exists ($param, $this->default))
+      throw new Exception (sprintf ("Configuration parameter '%s' not defined",
+                                    $param), 500);
+    // Create a conf where all the keys are defined. If the keys are already
+    // define, use them, or use the default ones
+    // Don't allow keys not defined in default ones
+    foreach ($this->default[$param] as $key=>$val)
     {
-      if (! is_array ($conf[$param]))
-        return $conf[$param];
-      // if the configuration is an array, check if all the keys are defined
-      // or use the default
-      foreach ($this->default[$param] as $key=>$val)
+      if ($key === "not configured")
       {
-        if (! isset ($conf[$param][$key]))
-          $conf[$param][$key] = $val;
-      }
-      foreach ($conf[$param] as $key=>$val)
-      {
-        if ($key === "not configured")
+        if (! array_key_exists ($param, $conf))
           continue;
-        if (is_array ($val))
+        foreach ($conf[$param] as $k=>$v)
         {
-          if (! isset ($this->default[$param]["not configured"]))
-            continue;
-          foreach ($this->default[$param]["not configured"] as $k=>$v)
-          {
-            if (! array_key_exists ($k, $conf[$param][$key]))
-              $conf[$param][$key][$k] = $v;
-          }
+          $conf[$param][$k] = array_replace_recursive ($val,
+                                                       $conf[$param][$k]);
         }
+        continue;
       }
-      if (count ($conf[$param]) > 1 && isset ($conf[$param]["not configured"]))
-        unset ($conf[$param]["not configured"]);
-      return $conf[$param];
+      if (! isset ($conf[$param][$key]))
+        $conf[$param][$key] = $val;
+      elseif (is_array ($val))
+        $conf[$param][$key] = array_replace_recursive ($val,
+                                                       $conf[$param][$key]);
     }
-    return $this->default[$param];
+    if (! array_key_exists ($param, $conf))
+      return $this->default[$param];
+    return $conf[$param];
   }
 
   /** Define a value for the parameter in the config file. Add all the default