diff --git a/jmock-2.5.1/src/org/jmock/lib/concurrent/DeterministicScheduler.java b/jmock-2.5.1/src/org/jmock/lib/concurrent/DeterministicScheduler.java
index 6dbc020..0dce41c 100644
--- a/jmock-2.5.1/src/org/jmock/lib/concurrent/DeterministicScheduler.java
+++ b/jmock-2.5.1/src/org/jmock/lib/concurrent/DeterministicScheduler.java
@@ -110,21 +110,21 @@ public class DeterministicScheduler implements ScheduledExecutorService {
         throw blockingOperationsNotSupported();
     }
 
-    public <T> List<Future<T>> invokeAll(Collection<Callable<T>> tasks) throws InterruptedException {
+    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) throws InterruptedException {
         throw blockingOperationsNotSupported();
     }
 
-    public <T> List<Future<T>> invokeAll(Collection<Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException {
+    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException {
         throw blockingOperationsNotSupported();
     }
     
-    public <T> T invokeAny(Collection<Callable<T>> tasks)
+    public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
         throws InterruptedException, ExecutionException 
     {
         throw blockingOperationsNotSupported();
     }
 
-    public <T> T invokeAny(Collection<Callable<T>> tasks, long timeout, TimeUnit unit) 
+    public <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) 
         throws InterruptedException, ExecutionException, TimeoutException 
     {
         throw blockingOperationsNotSupported();