diff --git a/src/main/java/com/samskivert/jdbc/LiaisonRegistry.java b/src/main/java/com/samskivert/jdbc/LiaisonRegistry.java
index a94d27e9..f3625219 100644
--- a/src/main/java/com/samskivert/jdbc/LiaisonRegistry.java
+++ b/src/main/java/com/samskivert/jdbc/LiaisonRegistry.java
@@ -25,6 +25,7 @@ public class LiaisonRegistry
      */
     public static DatabaseLiaison getLiaison (String url)
     {
+        if (url == null) throw new NullPointerException("URL must not be null");
         // see if we already have a liaison mapped for this connection
         DatabaseLiaison liaison = _mappings.get(url);
         if (liaison == null) {