diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/calendars/entities/BaseCalendar.java b/navalplanner-business/src/main/java/org/navalplanner/business/calendars/entities/BaseCalendar.java
index 235075275..c28cbc615 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/calendars/entities/BaseCalendar.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/calendars/entities/BaseCalendar.java
@@ -96,11 +96,17 @@ public class BaseCalendar extends BaseEntity implements IValidable {
         return hours;
     }
 
-    public void setHours(Days day, Integer hours) {
+    public void setHours(Days day, Integer hours)
+            throws IllegalArgumentException {
         setHoursForDay(day, hours);
     }
 
-    private void setHoursForDay(Days day, Integer hours) {
+    private void setHoursForDay(Days day, Integer hours)
+            throws IllegalArgumentException {
+        if ((hours != null) && (hours < 0)) {
+            throw new IllegalArgumentException(
+                    "The number of hours for a day can not be negative");
+        }
         hoursPerDay.put(day.ordinal(), hours);
     }
 
diff --git a/navalplanner-business/src/test/java/org/navalplanner/business/test/calendars/entities/BaseCalendarTest.java b/navalplanner-business/src/test/java/org/navalplanner/business/test/calendars/entities/BaseCalendarTest.java
index d758ddf01..c2686f8ac 100644
--- a/navalplanner-business/src/test/java/org/navalplanner/business/test/calendars/entities/BaseCalendarTest.java
+++ b/navalplanner-business/src/test/java/org/navalplanner/business/test/calendars/entities/BaseCalendarTest.java
@@ -532,4 +532,11 @@ public class BaseCalendarTest {
         assertThat(copy.getPreviousCalendar(), nullValue());
     }
 
+    @Test(expected = IllegalArgumentException.class)
+    public void testSetHoursInvalid() {
+        BaseCalendar calendar = createBasicCalendar();
+
+        calendar.setHours(Days.MONDAY, -5);
+    }
+
 }
diff --git a/navalplanner-webapp/src/main/java/org/navalplanner/web/calendars/BaseCalendarCRUDController.java b/navalplanner-webapp/src/main/java/org/navalplanner/web/calendars/BaseCalendarCRUDController.java
index f0a35ab42..4290deb46 100644
--- a/navalplanner-webapp/src/main/java/org/navalplanner/web/calendars/BaseCalendarCRUDController.java
+++ b/navalplanner-webapp/src/main/java/org/navalplanner/web/calendars/BaseCalendarCRUDController.java
@@ -264,7 +264,8 @@ public class BaseCalendarCRUDController extends GenericForwardComposer {
             item.appendChild(labelListcell);
 
             Listcell hoursListcell = new Listcell();
-            Intbox hoursIntbox = Util.bind(new Intbox(),
+            final Intbox intBox = new Intbox();
+            Intbox hoursIntbox = Util.bind(intBox,
                     new Util.Getter<Integer>() {
 
                         @Override
@@ -275,8 +276,12 @@ public class BaseCalendarCRUDController extends GenericForwardComposer {
 
                         @Override
                         public void set(Integer value) {
-                            baseCalendarModel.setHours(day,
-                                    value);
+                            try {
+                                baseCalendarModel.setHours(day, value);
+                            } catch (IllegalArgumentException e) {
+                                throw new WrongValueException(intBox, e
+                                        .getMessage());
+                            }
                         }
                     });