Index: Driver/Source/Connection.cs
===================================================================
--- Driver/Source/Connection.cs	(revision 1267)
+++ Driver/Source/Connection.cs	(working copy)
@@ -248,14 +248,16 @@
                         "Not allowed to change the 'ConnectionString' property while the connection (state=" + State +
                         ").");
 
+                string str_val = (null != value ? value : "");
+
                 MySqlConnectionStringBuilder newSettings;
                 lock (connectionStringCache)
                 {
-                    newSettings = (MySqlConnectionStringBuilder)connectionStringCache[value];
+                    newSettings = (MySqlConnectionStringBuilder)connectionStringCache[str_val];
                     if (null == newSettings)
                     {
-                        newSettings = new MySqlConnectionStringBuilder(value);
-                        connectionStringCache.Add(value, newSettings);
+                        newSettings = new MySqlConnectionStringBuilder(str_val);
+                        connectionStringCache.Add(str_val, newSettings);
                     }
                 }