Always provide a core loader to which the classloader task can add.
https://issues.apache.org/bugzilla/show_bug.cgi?id=47003
Patch licensed to the Apache Software Foundation.
2009-04-09 Martin von Gagern

Index: src/main/org/apache/tools/ant/Main.java
===================================================================
--- src/main/org/apache/tools/ant/Main.java	(revision 763549)
+++ src/main/org/apache/tools/ant/Main.java	(working copy)
@@ -690,6 +690,8 @@
         }
 
         final Project project = new Project();
+        if (coreLoader == null || coreLoader instanceof AntClassLoader == false)
+            coreLoader = new AntClassLoader(coreLoader, project, null, true);
         project.setCoreLoader(coreLoader);
 
         Throwable error = null;
Index: src/main/org/apache/tools/ant/taskdefs/Classloader.java
===================================================================
--- src/main/org/apache/tools/ant/taskdefs/Classloader.java	(revision 763549)
+++ src/main/org/apache/tools/ant/taskdefs/Classloader.java	(working copy)
@@ -169,9 +169,18 @@
                 return;
             }
 
-            String loaderName = (name == null) ? SYSTEM_LOADER_REF : name;
+            String loaderName;
+            Object obj;
+            if (name == null || SYSTEM_LOADER_REF.equals(name)) {
+                name = null;
+                loaderName = SYSTEM_LOADER_REF;
+                obj = getProject().getCoreLoader();
+            }
+            else {
+                loaderName = name;
+                obj = getProject().getReference(loaderName);
+            }
 
-            Object obj = getProject().getReference(loaderName);
             if (reset) {
                 // Are any other references held ? Can we 'close' the loader
                 // so it removes the locks on jars ?