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 09a17511b..61bc7b60d 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
@@ -408,6 +408,13 @@ public class Task extends TaskElement implements ITaskPositionConstrained {
 
     private static abstract class AllocationModificationStrategy {
 
+        protected final IResourceDAO resourceDAO;
+
+        public AllocationModificationStrategy(IResourceDAO resourceDAO) {
+            Validate.notNull(resourceDAO);
+            this.resourceDAO = resourceDAO;
+        }
+
         public abstract List<ResourcesPerDayModification> getResourcesPerDayModified(
                 List<ResourceAllocation<?>> allocations);
 
@@ -419,26 +426,30 @@ public class Task extends TaskElement implements ITaskPositionConstrained {
     private static class WithTheSameHoursAndResourcesPerDay extends
             AllocationModificationStrategy {
 
+        public WithTheSameHoursAndResourcesPerDay(IResourceDAO resourceDAO) {
+            super(resourceDAO);
+        }
+
         @Override
         public List<HoursModification> getHoursModified(
                 List<ResourceAllocation<?>> allocations) {
-            return HoursModification.fromExistent(allocations);
+            return HoursModification.fromExistent(allocations, resourceDAO);
         }
 
         @Override
         public List<ResourcesPerDayModification> getResourcesPerDayModified(
                 List<ResourceAllocation<?>> allocations) {
-            return ResourcesPerDayModification.fromExistent(allocations);
+            return ResourcesPerDayModification.fromExistent(allocations,
+                    resourceDAO);
         }
 
     }
 
     private static class WithAnotherResources extends
             AllocationModificationStrategy {
-        private final IResourceDAO resourceDAO;
 
-        WithAnotherResources(IResourceDAO resourceDAO) {
-            this.resourceDAO = resourceDAO;
+        public WithAnotherResources(IResourceDAO resourceDAO) {
+            super(resourceDAO);
         }
 
         @Override
@@ -479,7 +490,7 @@ public class Task extends TaskElement implements ITaskPositionConstrained {
 
             private void doReassignment(Direction direction) {
                 reassign(scenario, direction,
-                        new WithTheSameHoursAndResourcesPerDay());
+                        new WithTheSameHoursAndResourcesPerDay(resourceDAO));
             }
 
             @Override
diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/allocationalgorithms/AllocationModification.java b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/allocationalgorithms/AllocationModification.java
index bba5b9834..84a0b90f5 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/allocationalgorithms/AllocationModification.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/allocationalgorithms/AllocationModification.java
@@ -42,6 +42,25 @@ public abstract class AllocationModification {
         return result;
     }
 
+    /**
+     * It ensures that the provided allocations have at least one associated
+     * resource. A {@link AllocationModification} doesn't have associated
+     * resources when creating an {@link AllocationModification} from a
+     * unsatisfied generic allocation.
+     */
+    protected static <T extends AllocationModification> List<T> ensureNoOneWithoutAssociatedResources(
+            Collection<? extends T> modifications, IResourceDAO resourceDAO) {
+        List<T> result = new ArrayList<T>();
+        for (T each : modifications) {
+            if (each.hasNoResources()) {
+                each.withNewResources(resourceDAO);
+            }
+            assert !each.hasNoResources();
+            result.add(each);
+        }
+        return result;
+    }
+
     private final ResourceAllocation<?> beingModified;
 
     private List<Resource> resourcesOnWhichApplyAllocation;
@@ -52,6 +71,10 @@ public abstract class AllocationModification {
                 .unmodifiableList(new ArrayList<Resource>(resources));
     }
 
+    private boolean hasNoResources() {
+        return resourcesOnWhichApplyAllocation.isEmpty();
+    }
+
     protected void withNewResources(IResourceDAO resourceDAO) {
         resourcesOnWhichApplyAllocation = beingModified
                 .querySuitableResources(resourceDAO);
diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/allocationalgorithms/HoursModification.java b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/allocationalgorithms/HoursModification.java
index c1a45b6a0..99bd5e583 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/allocationalgorithms/HoursModification.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/allocationalgorithms/HoursModification.java
@@ -99,17 +99,18 @@ public abstract class HoursModification extends AllocationModification {
     }
 
     public static List<HoursModification> fromExistent(
-            Collection<? extends ResourceAllocation<?>> allocations) {
+            Collection<? extends ResourceAllocation<?>> allocations,
+            IResourceDAO resourceDAO) {
         List<HoursModification> result = new ArrayList<HoursModification>();
         for (ResourceAllocation<?> resourceAllocation : allocations) {
             result.add(resourceAllocation.asHoursModification());
         }
-        return result;
+        return ensureNoOneWithoutAssociatedResources(result, resourceDAO);
     }
 
     public static List<HoursModification> withNewResources(
             List<ResourceAllocation<?>> allocations, IResourceDAO resourceDAO) {
-        List<HoursModification> result = fromExistent(allocations);
+        List<HoursModification> result = fromExistent(allocations, resourceDAO);
         for (HoursModification each : result) {
             each.withNewResources(resourceDAO);
         }
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 f61abb510..fd09aa289 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
@@ -27,7 +27,6 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 
-import org.apache.commons.lang.Validate;
 import org.joda.time.LocalDate;
 import org.navalplanner.business.calendars.entities.AvailabilityTimeLine;
 import org.navalplanner.business.calendars.entities.CombinedWorkHours;
@@ -59,7 +58,6 @@ public abstract class ResourcesPerDayModification extends
                 ResourcesPerDay resourcesPerDay,
                 Collection<? extends Resource> resources) {
             super(resourceAllocation, resourcesPerDay, resources);
-            Validate.isTrue(!resources.isEmpty());
             this.genericAllocation = resourceAllocation;
         }
 
@@ -196,7 +194,8 @@ public abstract class ResourcesPerDayModification extends
 
     public static List<ResourcesPerDayModification> withNewResources(
             List<ResourceAllocation<?>> allocations, IResourceDAO resourceDAO) {
-        List<ResourcesPerDayModification> result = fromExistent(allocations);
+        List<ResourcesPerDayModification> result = fromExistent(allocations,
+                resourceDAO);
         for (ResourcesPerDayModification each : result) {
             each.withNewResources(resourceDAO);
         }
@@ -212,12 +211,13 @@ public abstract class ResourcesPerDayModification extends
     }
 
     public static List<ResourcesPerDayModification> fromExistent(
-            Collection<? extends ResourceAllocation<?>> allocations) {
+            Collection<? extends ResourceAllocation<?>> allocations,
+            IResourceDAO resourcesDAO) {
         List<ResourcesPerDayModification> result = new ArrayList<ResourcesPerDayModification>();
         for (ResourceAllocation<?> resourceAllocation : allocations) {
             result.add(resourceAllocation.asResourcesPerDayModification());
         }
-        return result;
+        return ensureNoOneWithoutAssociatedResources(result, resourcesDAO);
     }
 
     protected static ICalendar calendarFor(Resource associatedResource) {