diff --git a/src/main/java/seedu/address/commons/core/Messages.java b/src/main/java/seedu/address/commons/core/Messages.java
index 496fd9d21f80..6581c60754df 100644
--- a/src/main/java/seedu/address/commons/core/Messages.java
+++ b/src/main/java/seedu/address/commons/core/Messages.java
@@ -12,7 +12,7 @@ public class Messages {
     public static final String MESSAGE_INVALID_TASK_DISPLAYED_INDEX = "The task index provided is invalid";
     public static final String MESSAGE_COMPLETED_TASK = "The task has completed already...";
     public static final String MESSAGE_TASKS_LISTED_OVERVIEW = "%1$d tasks listed!";
-    public static final String MESSAGE_ZERO_HOURS_COMPLETION = "It is impossible to complete it in 0 hours ;)";
+    public static final String MESSAGE_ZERO_HOURS_COMPLETION = "It is impossible to complete it in less than 1 hour ;)";
     public static final String MESSAGE_INVALID_DEADLINE = "The date selected does not exist";
     public static final String MESSAGE_DEADLINE_CONTAINS_ILLEGAL_CHARACTERS = "Input deadline "
             + "contains illegal characters";
diff --git a/src/main/java/seedu/address/storage/XmlAdaptedTask.java b/src/main/java/seedu/address/storage/XmlAdaptedTask.java
index ab28a714f0e6..4ada3cfed2f4 100644
--- a/src/main/java/seedu/address/storage/XmlAdaptedTask.java
+++ b/src/main/java/seedu/address/storage/XmlAdaptedTask.java
@@ -1,5 +1,6 @@
 package seedu.address.storage;
 
+import static seedu.address.commons.core.Messages.MESSAGE_ZERO_HOURS_COMPLETION;
 import static seedu.address.commons.util.StringUtil.isNonZeroUnsignedInteger;
 import static seedu.address.logic.parser.ParserUtil.MESSAGE_INVALID_HOURS;
 
@@ -184,7 +185,7 @@ public Task toModelType() throws IllegalValueException {
 
         // Check validity of completed num of hours
         final int modelCompletedNumOfHours;
-        if (!completedNumOfHours.equals("-1") || Integer.valueOf(completedNumOfHours) < -1) {
+        if (completedNumOfHours == null) {
             throw new IllegalValueException(String.format(MISSING_FIELD_MESSAGE_FORMAT,
                     "Number of hours taken to complete"));
         } else {
@@ -195,6 +196,12 @@ public Task toModelType() throws IllegalValueException {
         //Boolean cannot be checked for null --> if (isCompleted == null)
         final boolean modelIsCompleted = isCompleted;
 
+        if (isCompleted && modelCompletedNumOfHours <= 0) {
+            throw new IllegalValueException(MESSAGE_ZERO_HOURS_COMPLETION);
+        } else if (!isCompleted && modelCompletedNumOfHours > 0) {
+            throw new IllegalValueException("Task is not completed yet ...");
+        }
+
         // Check validity of Milestones
         final List<Milestone> milestoneEntries = new ArrayList<Milestone>();
         if (milestonelist != null && !milestonelist.isEmpty()) {