diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/Task.java b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/Task.java
index 7e9aeebf9..dfd437639 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/Task.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/Task.java
@@ -29,6 +29,8 @@ import java.util.List;
 import java.util.Set;
 
 import org.apache.commons.lang.Validate;
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 import org.hibernate.validator.AssertTrue;
 import org.joda.time.DateTime;
 import org.joda.time.Days;
@@ -48,6 +50,8 @@ import org.navalplanner.business.resources.entities.Worker;
  */
 public class Task extends TaskElement {
 
+    private static final Log LOG = LogFactory.getLog(Task.class);
+
     public static Task createTask(TaskSource taskSource) {
         Task task = new Task();
         OrderElement orderElement = taskSource.getOrderElement();
@@ -104,7 +108,21 @@ public class Task extends TaskElement {
     }
 
     public Set<ResourceAllocation<?>> getResourceAllocations() {
-        return Collections.unmodifiableSet(resourceAllocations);
+        return Collections.unmodifiableSet(filterEmpty(resourceAllocations));
+    }
+
+    private Set<ResourceAllocation<?>> filterEmpty(
+            Set<ResourceAllocation<?>> allocations) {
+        Set<ResourceAllocation<?>> result = new HashSet<ResourceAllocation<?>>();
+        for (ResourceAllocation<?> each : allocations) {
+            if (each.hasAssignments()) {
+                result.add(each);
+            } else {
+                LOG.warn("there is an empty resource allocation: "
+                        + each.toString() + " ,on task: " + this);
+            }
+        }
+        return result;
     }
 
     public void addResourceAllocation(ResourceAllocation<?> resourceAllocation) {
@@ -112,8 +130,13 @@ public class Task extends TaskElement {
             throw new IllegalArgumentException(
                     "the resourceAllocation's task must be this task");
         }
-        resourceAllocations.add(resourceAllocation);
-        resourceAllocation.associateAssignmentsToResource();
+        if (resourceAllocation.hasAssignments()) {
+            resourceAllocations.add(resourceAllocation);
+            resourceAllocation.associateAssignmentsToResource();
+        } else {
+            LOG.warn("adding a resource allocation without assignments: "
+                    + resourceAllocation);
+        }
     }
 
     public void removeResourceAllocation(
@@ -289,7 +312,7 @@ public class Task extends TaskElement {
     @Override
     protected void moveAllocations() {
         List<ModifiedAllocation> copied = ModifiedAllocation
-                .copy(resourceAllocations);
+                .copy(getResourceAllocations());
         List<ResourcesPerDayModification> allocations = ResourcesPerDayModification
                 .fromExistent(ModifiedAllocation.modified(copied));
         if (allocations.isEmpty()) {
diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/allocationalgorithms/ResourcesPerDayModification.java b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/allocationalgorithms/ResourcesPerDayModification.java
index d968b788c..2f8a18ac9 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/allocationalgorithms/ResourcesPerDayModification.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/allocationalgorithms/ResourcesPerDayModification.java
@@ -119,6 +119,7 @@ public abstract class ResourcesPerDayModification extends
             Collection<? extends ResourceAllocation<?>> allocations) {
         List<ResourcesPerDayModification> result = new ArrayList<ResourcesPerDayModification>();
         for (ResourceAllocation<?> resourceAllocation : allocations) {
+            Validate.isTrue(resourceAllocation.hasAssignments());
             ResourcesPerDay perDay = resourceAllocation
                     .getResourcesPerDay();
             Validate.notNull(perDay);
diff --git a/navalplanner-business/src/test/java/org/navalplanner/business/test/planner/entities/TaskTest.java b/navalplanner-business/src/test/java/org/navalplanner/business/test/planner/entities/TaskTest.java
index 328a8e93b..284d69634 100644
--- a/navalplanner-business/src/test/java/org/navalplanner/business/test/planner/entities/TaskTest.java
+++ b/navalplanner-business/src/test/java/org/navalplanner/business/test/planner/entities/TaskTest.java
@@ -34,7 +34,6 @@ import org.junit.Test;
 import org.navalplanner.business.orders.entities.HoursGroup;
 import org.navalplanner.business.orders.entities.OrderLine;
 import org.navalplanner.business.orders.entities.TaskSource;
-import org.navalplanner.business.planner.entities.ResourceAllocation;
 import org.navalplanner.business.planner.entities.SpecificResourceAllocation;
 import org.navalplanner.business.planner.entities.Task;
 import org.navalplanner.business.planner.entities.TaskElement;
@@ -80,23 +79,31 @@ public class TaskTest {
     }
 
     @Test
-    public void taskAddResourceAllocation() {
+    public void addingEmptyResourceAllocationDoesntAddIt() {
         assertThat(task.getResourceAllocations().size(), equalTo(0));
 
         SpecificResourceAllocation resourceAllocation = SpecificResourceAllocation.create(task);
         task.addResourceAllocation(resourceAllocation);
 
+        assertThat(task.getResourceAllocations().size(), equalTo(0));
+    }
+
+    @Test
+    public void addingNoEmptyResourceAllocationAddsIt() {
+        assertThat(task.getResourceAllocations().size(), equalTo(0));
+
+        SpecificResourceAllocation resourceAllocation = stubResourceAllocationWithAssignedHours(
+                task, 500);
+        task.addResourceAllocation(resourceAllocation);
         assertThat(task.getResourceAllocations().size(), equalTo(1));
-        assertThat(
-                resourceAllocation.getTask().getResourceAllocations().size(),
-                equalTo(1));
     }
 
     @Test
     public void taskRemoveResourceAllocation() {
         assertThat(task.getResourceAllocations().size(), equalTo(0));
 
-        SpecificResourceAllocation resourceAllocation = SpecificResourceAllocation.create(task);
+        SpecificResourceAllocation resourceAllocation = stubResourceAllocationWithAssignedHours(
+                task, 500);
         task.addResourceAllocation(resourceAllocation);
 
         assertThat(task.getResourceAllocations().size(), equalTo(1));
@@ -112,17 +119,21 @@ public class TaskTest {
 
     @Test
     public void aTaskWithAllocationsReturnsTheSumOfItsAllocations() {
-        task.addResourceAllocation(stubResourceAllocationWithAssignedHours(5));
-        task.addResourceAllocation(stubResourceAllocationWithAssignedHours(3));
+        task.addResourceAllocation(stubResourceAllocationWithAssignedHours(
+                task, 5));
+        task.addResourceAllocation(stubResourceAllocationWithAssignedHours(
+                task, 3));
         assertThat(task.getAssignedHours(), equalTo(8));
     }
 
-    private ResourceAllocation<?> stubResourceAllocationWithAssignedHours(
+    private SpecificResourceAllocation stubResourceAllocationWithAssignedHours(
+            Task task,
             int hours) {
-        ResourceAllocation<?> resourceAllocation = createNiceMock(ResourceAllocation.class);
+        SpecificResourceAllocation resourceAllocation = createNiceMock(SpecificResourceAllocation.class);
         expect(resourceAllocation.getAssignedHours()).andReturn(hours)
                 .anyTimes();
         expect(resourceAllocation.getTask()).andReturn(task).anyTimes();
+        expect(resourceAllocation.hasAssignments()).andReturn(true).anyTimes();
         replay(resourceAllocation);
         return resourceAllocation;
     }