diff --git a/data/duke.txt b/data/duke.txt
index fa1502ebfd..e69de29bb2 100644
--- a/data/duke.txt
+++ b/data/duke.txt
@@ -1,6 +0,0 @@
-<<<<<<< HEAD
-D|0|abc|2022-10-04T12:00
-=======
-T|1|play
-T|1|eat
->>>>>>> 214560c1796b2e9cd7580c42ade8f2d41082b479
diff --git a/src/main/java/Duke/Parser.java b/src/main/java/Duke/Parser.java
index 1869b7e61d..f1c7525b6e 100644
--- a/src/main/java/Duke/Parser.java
+++ b/src/main/java/Duke/Parser.java
@@ -33,10 +33,12 @@ public static void readLine(Ui ui, String command, TaskList tasks, MainWindow ma
     }
 
     private static void markUnmarkTask(String[] strArr, TaskList tasks) {
-        assert java.util.regex.Pattern.matches("\\d+", strArr[1]) : "The 2nd parameter should be a " +
-                                                                    "positive integer";
+        assert java.util.regex.Pattern.matches("\\d+", strArr[1]) : "The 2nd parameter should be a "
+                                                                          + "positive integer";
         int index = Integer.parseInt(strArr[1]) - 1;
-        assert index < tasks.getSize() : "the index to be mark or unmark should be less than " + tasks.getSize();
+        assert index != -1 : "the 2nd parameter should start from index 1";
+        assert index < tasks.getSize() : "the 2nd parameter to be mark or unmark should be less than or equals to "
+                                         + tasks.getSize();
         if (strArr[0].equals("mark")) {
             tasks.markTaskAsDone(index);
         } else if (strArr[0].equals("unmark")) {
@@ -45,10 +47,11 @@ private static void markUnmarkTask(String[] strArr, TaskList tasks) {
     }
 
     private static void deleteTask(String[] strArr, TaskList tasks) {
-        assert java.util.regex.Pattern.matches("\\d+", strArr[1]) : "The 2nd parameter should be a " +
-                                                                     "positive integer";
+        assert java.util.regex.Pattern.matches("\\d+", strArr[1]) : "The 2nd parameter should be a "
+                                                                          + "positive integer";
         int index = Integer.parseInt(strArr[1]) - 1;
-        assert index < tasks.getSize() : "the index to be deleted should be less than " + tasks.getSize();
+        assert index != -1 : "the 2nd parameter should start from index 1";
+        assert index < tasks.getSize() : "the 2nd parameter should be less than or equals to " + tasks.getSize();
         tasks.deleteTask(index);
     }