diff --git a/src/main/java/awex/Storage.java b/src/main/java/awex/Storage.java
index 129b5cbe73..1f85eac258 100644
--- a/src/main/java/awex/Storage.java
+++ b/src/main/java/awex/Storage.java
@@ -18,6 +18,7 @@ public File load() throws IOException {
     public void store(TaskList t) throws IOException {
         FileWriter fw = new FileWriter(this.filepath);
         int len = t.size();
+        assert len >= 0;
         String str = "";
         for (int i = 0; i < len; i++) {
             str += t.get(i).toString() + "\n";
diff --git a/src/main/java/awex/TaskList.java b/src/main/java/awex/TaskList.java
index 5ce561c7ae..ed8d0b8c22 100644
--- a/src/main/java/awex/TaskList.java
+++ b/src/main/java/awex/TaskList.java
@@ -42,6 +42,7 @@ public void add(Task t) {
     public LinkedList<Task> find(String str) {
         LinkedList<Task> list = new LinkedList<>();
         int len = this.tasks.size();
+        assert len >= 0;
         for (int i = 0; i < len; i++) {
             Task t = this.tasks.get(i);
             if (t.isPartOfDesc(str)) {
diff --git a/src/main/java/awex/Ui.java b/src/main/java/awex/Ui.java
index fc7759256d..8b359f555d 100644
--- a/src/main/java/awex/Ui.java
+++ b/src/main/java/awex/Ui.java
@@ -54,6 +54,7 @@ public String emptyListMessage() {
     public String showListMessage(TaskList list) {
         String str = "Here are the tasks in your list:\n";
         int len = list.size();
+        assert len >= 0;
         for (int i = 1; i <= len; i++) {
             str += i + "." + list.get(i - 1).showAll() + "\n";
         }
@@ -70,6 +71,7 @@ public String showFindMessage(TaskList tasks, String str) {
         String stri = "Here are the matching tasks in your list:\n";
         LinkedList<Task> list = tasks.find(str);
         int len = list.size();
+        assert len >= 0;
         for (int i = 1; i <= len; i++) {
             stri += i + "." + list.get(i - 1).showAll() + "\n";
         }
@@ -87,6 +89,7 @@ public String wrongMarkDeleteFormatMessage(String str) {
      * Prints explainer message when user provides inaccessible list index.
      */
     public String wrongIndexMessage(int i, int len) {
+        assert len >= 0;
         if (i == 0) {
             return "Pick a value between 1 and " + len + ".";
         } else {
diff --git a/src/main/java/tasks/Task.java b/src/main/java/tasks/Task.java
index 9c3b46c0a8..f2d1247c8a 100644
--- a/src/main/java/tasks/Task.java
+++ b/src/main/java/tasks/Task.java
@@ -16,6 +16,7 @@ public class Task {
     public Task(String what, String status, String type) {
         this.what = what;
         this.type = type;
+        assert status.equals("t") || status.equals("f");
         if (status.equals("t")) {
             this.status = "[X]";
         } else {
@@ -38,9 +39,10 @@ public String showAll() {
      * @param status completion status of task
      */
     public void changeStatus(String status) {
+        assert status.equals("mark") || status.equals("unmark");
         if (status.equals("mark")) {
             this.status = "[X]";
-        } else if (status.equals("unmark")) {
+        } else {
             this.status = "[ ]";
         }
     }