diff --git a/src/main/java/seedu/address/logic/commands/AddAppointmentCommand.java b/src/main/java/seedu/address/logic/commands/AddAppointmentCommand.java
index 24beb79b6e5..ae9cc7aa53a 100644
--- a/src/main/java/seedu/address/logic/commands/AddAppointmentCommand.java
+++ b/src/main/java/seedu/address/logic/commands/AddAppointmentCommand.java
@@ -11,6 +11,7 @@
 import seedu.address.model.Model;
 import seedu.address.model.appointment.Appointment;
 import seedu.address.model.appointment.exceptions.InvalidAppointmentException;
+import seedu.address.model.person.exceptions.PersonNotFoundException;
 
 
 /**
@@ -50,9 +51,12 @@ public CommandResult execute(Model model) throws CommandException {
         if (model.hasAppointment(toAdd)) {
             throw new CommandException(MESSAGE_DUPLICATE_APPOINTMENT);
         }
-
-        if (!model.isValidAppointment(toAdd)) {
-            throw new InvalidAppointmentException();
+        try {
+            if (!model.isValidAppointment(toAdd)) {
+                throw new InvalidAppointmentException();
+            }
+        } catch (PersonNotFoundException e) {
+            throw new CommandException("The provided Doctor / Patient is not registered in the system");
         }
 
         model.addAppointment(toAdd);
diff --git a/src/main/java/seedu/address/model/AddressBook.java b/src/main/java/seedu/address/model/AddressBook.java
index 4856b4ab480..bb1a3537305 100644
--- a/src/main/java/seedu/address/model/AddressBook.java
+++ b/src/main/java/seedu/address/model/AddressBook.java
@@ -12,6 +12,7 @@
 import seedu.address.model.person.Nric;
 import seedu.address.model.person.Person;
 import seedu.address.model.person.UniquePersonList;
+import seedu.address.model.person.exceptions.PersonNotFoundException;
 
 /**
  * Wraps all data at the address-book level
@@ -87,7 +88,7 @@ public boolean hasPerson(Person person) {
         return persons.contains(person);
     }
 
-    public Person getPersonByNric(Nric nricObj) {
+    public Person getPersonByNric(Nric nricObj) throws PersonNotFoundException {
         requireNonNull(nricObj);
         return persons.getPersonByNric(nricObj);
     }
diff --git a/src/main/java/seedu/address/model/ModelManager.java b/src/main/java/seedu/address/model/ModelManager.java
index 23cd912f9df..c04a138c957 100644
--- a/src/main/java/seedu/address/model/ModelManager.java
+++ b/src/main/java/seedu/address/model/ModelManager.java
@@ -15,6 +15,7 @@
 import seedu.address.model.person.Nric;
 import seedu.address.model.person.Person;
 import seedu.address.model.person.Type;
+import seedu.address.model.person.exceptions.PersonNotFoundException;
 
 /**
  * Represents the in-memory model of the address book data.
@@ -189,10 +190,9 @@ public boolean equals(Object other) {
      * @param appointment appointment to check validity of
      * @return boolean indicating if appointment is valid
      */
-    public boolean isValidAppointment(Appointment appointment) {
+    public boolean isValidAppointment(Appointment appointment) throws PersonNotFoundException {
         Nric doctorNric = appointment.getDoctorNric();
         Nric patientNric = appointment.getPatientNric();
-
         Person doctor = addressBook.getPersonByNric(doctorNric);
         Person patient = addressBook.getPersonByNric(patientNric);
 
diff --git a/src/main/java/seedu/address/model/person/UniquePersonList.java b/src/main/java/seedu/address/model/person/UniquePersonList.java
index 414ebb515dd..d810b02770c 100644
--- a/src/main/java/seedu/address/model/person/UniquePersonList.java
+++ b/src/main/java/seedu/address/model/person/UniquePersonList.java
@@ -160,7 +160,7 @@ private boolean personsAreUnique(List<Person> persons) {
         return true;
     }
 
-    public Person getPersonByNric(Nric nricObj) {
+    public Person getPersonByNric(Nric nricObj) throws PersonNotFoundException {
         ArrayList<Person> personList = new ArrayList<Person>(internalList);
 
         for (Person p : personList) {
diff --git a/src/test/java/seedu/address/logic/commands/AddAppointmentCommandTest.java b/src/test/java/seedu/address/logic/commands/AddAppointmentCommandTest.java
index 4a022807203..796ac41cf5a 100644
--- a/src/test/java/seedu/address/logic/commands/AddAppointmentCommandTest.java
+++ b/src/test/java/seedu/address/logic/commands/AddAppointmentCommandTest.java
@@ -30,6 +30,13 @@ void execute_validCommand_executesCommand() throws CommandException, ParseExcept
         assertTrue(modelManager.getFilteredAppointmentList().size() == 1);
     }
 
+    @Test
+    void execute_invalidCommand_missingPerson() throws CommandException, ParseException {
+        Appointment appt = new Appointment(BROWN.getNric(), ALICE.getNric(), new AppointmentDate("2024-09-01"));
+        AddAppointmentCommand ad = new AddAppointmentCommand(appt);
+        assertThrows(CommandException.class, () -> ad.execute(modelManager));
+    }
+
     @Test
     void execute_invalidCommand_throwsInvalidAppointmentException() throws ParseException {
         modelManager.addPerson(ALICE);