diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/GenericResourceAllocation.java b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/GenericResourceAllocation.java
index 913f8d215..d75d4c503 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/GenericResourceAllocation.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/GenericResourceAllocation.java
@@ -284,12 +284,6 @@ public class GenericResourceAllocation extends
         return allocation;
     }
 
-    @Override
-    public ResourcesPerDayModification asResourcesPerDayModification() {
-        return ResourcesPerDayModification.create(this,
-                getIntendedResourcesPerDay(), getAssociatedResources());
-    }
-
     @Override
     public HoursModification asHoursModification() {
         return HoursModification.create(this, getIntendedHours(),
diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/ResourceAllocation.java b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/ResourceAllocation.java
index f1252f61d..364794a63 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/ResourceAllocation.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/ResourceAllocation.java
@@ -594,7 +594,21 @@ public abstract class ResourceAllocation<T extends DayAssignment> extends
     public abstract ResourcesPerDayModification withDesiredResourcesPerDay(
             ResourcesPerDay resourcesPerDay);
 
-    public abstract ResourcesPerDayModification asResourcesPerDayModification();
+    public final ResourcesPerDayModification asResourcesPerDayModification() {
+        if (getIntendedResourcesPerDay().isZero()) {
+            return null;
+        }
+        if (this instanceof GenericResourceAllocation) {
+            GenericResourceAllocation generic = (GenericResourceAllocation) this;
+            return ResourcesPerDayModification.create(generic,
+                    getIntendedResourcesPerDay(), getAssociatedResources());
+        } else if (this instanceof SpecificResourceAllocation) {
+            SpecificResourceAllocation specific = (SpecificResourceAllocation) this;
+            return ResourcesPerDayModification.create(specific,
+                    getIntendedResourcesPerDay());
+        }
+        throw new RuntimeException("can't handle: " + this.getClass());
+    }
 
     public abstract HoursModification asHoursModification();
 
diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/SpecificResourceAllocation.java b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/SpecificResourceAllocation.java
index f0afc3a2e..6c49de9ea 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/SpecificResourceAllocation.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/SpecificResourceAllocation.java
@@ -243,12 +243,6 @@ public class SpecificResourceAllocation extends
         return result;
     }
 
-    @Override
-    public ResourcesPerDayModification asResourcesPerDayModification() {
-        return ResourcesPerDayModification.create(this,
-                getIntendedResourcesPerDay());
-    }
-
     @Override
     public HoursModification asHoursModification() {
         return HoursModification.create(this, getIntendedHours());
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 eb6937afb..e369e35ca 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
@@ -723,6 +723,11 @@ public class Task extends TaskElement implements ITaskPositionConstrained {
             Direction direction, List<ResourceAllocation<?>> toBeModified) {
         List<ResourcesPerDayModification> allocations = strategy
                 .getResourcesPerDayModified(toBeModified);
+        if (allocations.isEmpty()) {
+            LOG.warn("all allocations for task " + this
+                    + " have no valid data that could be used");
+            return;
+        }
         switch (calculatedValue) {
         case NUMBER_OF_HOURS:
             ResourceAllocation.allocating(allocations).allocateOnTaskLength();
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 65b6f8f1e..799d17642 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
@@ -29,6 +29,7 @@ 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;
@@ -198,8 +199,9 @@ public abstract class ResourcesPerDayModification extends
     public static ResourcesPerDayModification create(
             GenericResourceAllocation resourceAllocation,
             ResourcesPerDay resourcesPerDay, List<? extends Resource> resources) {
-        return new OnGenericAllocation(resourceAllocation,
-                resourcesPerDay, resources);
+        Validate.isTrue(!resourcesPerDay.isZero());
+        return new OnGenericAllocation(resourceAllocation, resourcesPerDay,
+                resources);
     }
 
     public static List<ResourcesPerDayModification> withNewResources(
@@ -215,6 +217,7 @@ public abstract class ResourcesPerDayModification extends
     public static ResourcesPerDayModification create(
             SpecificResourceAllocation resourceAllocation,
             ResourcesPerDay resourcesPerDay) {
+        Validate.isTrue(!resourcesPerDay.isZero());
         return new OnSpecificAllocation(resourceAllocation,
                 resourcesPerDay, Collections.singletonList(resourceAllocation
                         .getResource()));
@@ -225,7 +228,11 @@ public abstract class ResourcesPerDayModification extends
             IResourceDAO resourcesDAO) {
         List<ResourcesPerDayModification> result = new ArrayList<ResourcesPerDayModification>();
         for (ResourceAllocation<?> resourceAllocation : allocations) {
-            result.add(resourceAllocation.asResourcesPerDayModification());
+            ResourcesPerDayModification modification = resourceAllocation
+                    .asResourcesPerDayModification();
+            if (modification != null) {
+                result.add(modification);
+            }
         }
         return ensureNoOneWithoutAssociatedResources(result, resourcesDAO);
     }