diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/StretchesFunction.java b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/StretchesFunction.java
index c9348077f..83c3b3a2d 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/StretchesFunction.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/StretchesFunction.java
@@ -352,7 +352,7 @@ public class StretchesFunction extends AssignmentFunction {
         return type == null ? Type.DEFAULT : type;
     }
 
-    private Type getDesiredType() {
+    public Type getDesiredType() {
         return desiredType == null ? getType() : desiredType;
     }
 
@@ -462,4 +462,8 @@ public class StretchesFunction extends AssignmentFunction {
         return left;
     }
 
+    public boolean ifInterpolatedMustHaveAtLeastTwoStreches() {
+        return getDesiredType() != Type.INTERPOLATED || stretches.size() >= 2;
+    }
+
 }
diff --git a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/IStretchesFunctionModel.java b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/IStretchesFunctionModel.java
index 8197e9796..a50983511 100644
--- a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/IStretchesFunctionModel.java
+++ b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/IStretchesFunctionModel.java
@@ -31,6 +31,7 @@ import org.navalplanner.business.planner.entities.AssignmentFunction;
 import org.navalplanner.business.planner.entities.Stretch;
 import org.navalplanner.business.planner.entities.StretchesFunction;
 import org.navalplanner.business.planner.entities.Task;
+import org.navalplanner.business.planner.entities.StretchesFunction.Type;
 
 
 
@@ -45,7 +46,7 @@ public interface IStretchesFunctionModel {
      * Initial conversation steps
      */
 
-    void init(StretchesFunction stretchesFunction, Task task);
+    void init(StretchesFunction stretchesFunction, Task task, Type type);
 
     /*
      * Intermediate conversation steps
diff --git a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StrechesFunctionConfiguration.java b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StrechesFunctionConfiguration.java
index 964dc0db0..336e5e076 100644
--- a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StrechesFunctionConfiguration.java
+++ b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StrechesFunctionConfiguration.java
@@ -49,7 +49,9 @@ public abstract class StrechesFunctionConfiguration implements
                 "/planner/stretches_function.zul",
                 getParentOnWhichOpenWindow(), args);
         Util.createBindingsFor(window);
-        stretchesFunctionController.setResourceAllocation(getAllocation());
+        ResourceAllocation<?> allocation = getAllocation();
+        stretchesFunctionController
+                .setResourceAllocation(allocation, getType());
         stretchesFunctionController.showWindow();
         getAllocation().setAssignmentFunction(
                 stretchesFunctionController.getAssignmentFunction());
@@ -83,7 +85,7 @@ public abstract class StrechesFunctionConfiguration implements
     public void applyDefaultFunction(ResourceAllocation<?> resourceAllocation) {
         StretchesFunction stretchesFunction = StretchesFunctionModel
                 .createDefaultStretchesFunction(resourceAllocation.getTask()
-                        .getEndDate(), getType());
+                        .getEndDate());
         resourceAllocation.setAssignmentFunction(stretchesFunction);
     }
 
diff --git a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StretchesFunctionController.java b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StretchesFunctionController.java
index 6e1a6696b..72467eeae 100644
--- a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StretchesFunctionController.java
+++ b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StretchesFunctionController.java
@@ -36,6 +36,7 @@ import org.navalplanner.business.planner.entities.ResourceAllocation;
 import org.navalplanner.business.planner.entities.Stretch;
 import org.navalplanner.business.planner.entities.StretchesFunction;
 import org.navalplanner.business.planner.entities.Task;
+import org.navalplanner.business.planner.entities.StretchesFunction.Type;
 import org.navalplanner.web.common.Util;
 import org.zkoss.zk.ui.Component;
 import org.zkoss.zk.ui.SuspendNotAllowedException;
@@ -77,12 +78,13 @@ public class StretchesFunctionController extends GenericForwardComposer {
         return stretchesFunctionModel.getStretchesFunction();
     }
 
-    public void setResourceAllocation(ResourceAllocation<?> resourceAllocation) {
+    public void setResourceAllocation(ResourceAllocation<?> resourceAllocation,
+            Type type) {
         AssignmentFunction assignmentFunction = resourceAllocation
                 .getAssignmentFunction();
         Task task = resourceAllocation.getTask();
         stretchesFunctionModel.init((StretchesFunction) assignmentFunction,
-                task);
+                task, type);
         reloadStretchesListAndCharts();
     }
 
diff --git a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StretchesFunctionModel.java b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StretchesFunctionModel.java
index 10531d705..16309dc2a 100644
--- a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StretchesFunctionModel.java
+++ b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StretchesFunctionModel.java
@@ -38,7 +38,6 @@ import org.navalplanner.business.planner.entities.AssignmentFunction;
 import org.navalplanner.business.planner.entities.Stretch;
 import org.navalplanner.business.planner.entities.StretchesFunction;
 import org.navalplanner.business.planner.entities.Task;
-import org.navalplanner.business.planner.entities.StretchesFunction.Type;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.context.annotation.Scope;
@@ -54,16 +53,13 @@ import org.springframework.transaction.annotation.Transactional;
 @Scope(BeanDefinition.SCOPE_PROTOTYPE)
 public class StretchesFunctionModel implements IStretchesFunctionModel {
 
-    public static StretchesFunction createDefaultStretchesFunction(
-            Date endDate, Type type) {
+    public static StretchesFunction createDefaultStretchesFunction(Date endDate) {
         StretchesFunction stretchesFunction = StretchesFunction.create();
-        stretchesFunction.changeTypeTo(type);
         Stretch stretch = new Stretch();
         stretch.setDate(new LocalDate(endDate));
         stretch.setLengthPercentage(BigDecimal.ONE);
         stretch.setAmountWorkPercentage(BigDecimal.ONE);
         stretchesFunction.addStretch(stretch);
-
         return stretchesFunction;
     }
 
@@ -89,12 +85,15 @@ public class StretchesFunctionModel implements IStretchesFunctionModel {
 
     @Override
     @Transactional(readOnly = true)
-    public void init(StretchesFunction stretchesFunction, Task task) {
+    public void init(
+            StretchesFunction stretchesFunction,
+            Task task,
+            org.navalplanner.business.planner.entities.StretchesFunction.Type type) {
         if (stretchesFunction != null) {
             assignmentFunctionDAO.reattach(stretchesFunction);
             this.originalStretchesFunction = stretchesFunction;
             this.stretchesFunction = stretchesFunction.copy();
-
+            this.stretchesFunction.changeTypeTo(type);
             this.task = task;
             forceLoadTask();
             this.taskEndDate = task.getEndDate();
@@ -133,10 +132,16 @@ public class StretchesFunctionModel implements IStretchesFunctionModel {
                         _("Last stretch should have one hundred percent for "
                                 + "length and amount of work percentage"));
             }
+            if (!stretchesFunction.ifInterpolatedMustHaveAtLeastTwoStreches()) {
+                throw new ValidationException(
+                        _("For interpolation at least two streches needed"));
+            }
 
             if (originalStretchesFunction != null) {
                 originalStretchesFunction
                         .resetToStrechesFrom(stretchesFunction);
+                originalStretchesFunction.changeTypeTo(stretchesFunction
+                                .getDesiredType());
                 stretchesFunction = originalStretchesFunction;
             }
         }