diff --git a/src/HL7/Message.php b/src/HL7/Message.php
index 9284f7e..9b571a5 100644
--- a/src/HL7/Message.php
+++ b/src/HL7/Message.php
@@ -239,7 +239,7 @@ public function removeSegmentsByName(string $segmentName): int
      */
     public function setSegment(Segment $segment, int $index): bool
     {
-        if (!isset($index) || $index > count($this->segments)) {
+        if ($index > count($this->segments)) {
             throw new InvalidArgumentException('Index out of range');
         }
 
diff --git a/tests/MessageTest.php b/tests/MessageTest.php
index 6e5ff1e..a065bac 100644
--- a/tests/MessageTest.php
+++ b/tests/MessageTest.php
@@ -15,6 +15,27 @@
 
 class MessageTest extends TestCase
 {
+    /** @test */
+    public function segments_can_be_retrieved_using_index(): void
+    {
+        $msg = new Message("MSH|^~\\&|1|\rABC|\r");
+        self::assertSame('ABC', $msg->getSegmentByIndex(1)->getName());
+    }
+
+    /** @test */
+    public function segmentByIndex_returns_null_when_target_index_is_beyond_the_total_number_of_segments(): void
+    {
+        $msg = new Message("MSH|^~\\&|1|\rABC|\r");
+        self::assertNull($msg->getSegmentByIndex(2));
+    }
+
+    /** @test */
+    public function it_throws_error_when_field_separator_control_char_doesnt_match_the_actual_field_separators(): void
+    {
+        $this->expectException(HL7Exception::class);
+        $msg = new Message("MSH|^~\\&#\r");
+    }
+
     /**
      * @test
      * @throws Exception
@@ -209,6 +230,28 @@ public function a_new_segment_can_be_inserted_between_two_existing_segments(): v
         self::assertSame('BBB', $msg->getSegmentByIndex(2)->getName(), 'Existing segment should shift');
     }
 
+    /** @test */
+    public function MSH_segment_can_be_inserted_with_new_control_characters(): void
+    {
+        $msg = new Message();
+        $msh = new MSH(['MSH', '#~\&', '', '', '', '', '', '', ['AAA', 'BBB'], '', '', '555']);
+        $msg->insertSegment($msh, 0);
+        self::assertSame('MSH', $msg->getSegmentByIndex(0)->getName(), 'Inserted MSH');
+        self::assertSame(
+            'MSH|#~\&|||||||AAA#BBB|||555|\n',
+            $msg->toString(),
+            'Component separator should be # and version should be 555'
+        );
+    }
+
+    /** @test */
+    public function insertSegment_appends_when_the_target_index_is_same_as_the_count_of_total_segments(): void
+    {
+        $msg = new Message("MSH|^~\\&|1|\rAAA|1", null, true);
+        $msg->insertSegment(new Segment('XXX'), 2);
+        self::assertSame('MSH|^~\&|1|\nAAA|1|\nXXX|\n', $msg->toString());
+    }
+
     /** @test */
     public function it_should_not_be_possible_to_insert_segment_beyond_last_index(): void
     {
@@ -228,6 +271,27 @@ public function a_segment_can_be_overwritten(): void
         self::assertSame('BBB', $msg->getSegmentByIndex(0)->getName(), 'BBB should have replaced AAA');
     }
 
+    /** @test */
+    public function setSegment_throws_exception_when_the_target_index_is_beyond_total_count_of_segments(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $msg = new Message("MSH|^~\\&|1|\nAAA|1|\n");
+        $msg->setSegment(new Segment('BBB'), 3);
+    }
+
+    /** @test */
+    public function setSegment_resets_control_characters_when_adding_MSH_segment_at_0th_index(): void
+    {
+        $msg = new Message();
+        $msh = new MSH(['MSH', '#~\&', '', '', '', '', '', '', ['AAA', 'BBB'], '', '', '555']);
+        $msg->setSegment($msh, 0);
+        self::assertSame(
+            'MSH|#~\&|||||||AAA#BBB|||555|\n',
+            $msg->toString(),
+            'Component separator should be # and version should be 555'
+        );
+    }
+
     /** @test */
     public function same_segment_type_can_be_added_multiple_times(): void
     {
@@ -291,6 +355,27 @@ public function a_segment_can_be_retrieved_as_a_string(): void
         self::assertSame('a^b1&b2^c', $msg->getSegmentFieldAsString(1, 2), 'XXX(2) as string');
     }
 
+    /** @test */
+    public function getSegmentAsString_returns_null_if_the_target_segment_doesnt_exist(): void
+    {
+        $msg = new Message("MSH|^~\\&|1|\nABC|");
+        self::assertNull($msg->getSegmentAsString(5));
+    }
+
+    /** @test */
+    public function getSegmentFieldAsString_returns_null_if_the_target_segment_doesnt_exist(): void
+    {
+        $msg = new Message("MSH|^~\\&|1|\nABC|");
+        self::assertNull($msg->getSegmentFieldAsString(5, 1));
+    }
+
+    /** @test */
+    public function getSegmentFieldAsString_returns_null_if_the_target_field_doesnt_exist(): void
+    {
+        $msg = new Message("MSH|^~\\&|1|\nABC|");
+        self::assertNull($msg->getSegmentFieldAsString(1, 5));
+    }
+
     /**
      * @test
      */
@@ -325,6 +410,13 @@ public function segment_index_can_be_retrieved_from_a_message(): void
         self::assertSame(2, $message->getSegmentIndex($pid));
     }
 
+    /** @test */
+    public function segmentIndex_returns_null_if_the_target_segment_is_not_found(): void
+    {
+        $message = new Message("MSH|^~\\&|1|\nABC|\n");
+        self::assertNull($message->getSegmentIndex(new Segment('XXX')));
+    }
+
     /** @test */
     public function message_type_can_be_checked(): void
     {
@@ -342,6 +434,11 @@ public function message_type_can_be_checked(): void
         self::assertTrue($msg->isSiu());
         self::assertFalse($msg->isOrm());
         self::assertFalse($msg->isOru());
+
+        $msg = new Message("MSH|^~\&|||||||ADT^A01|");
+        self::assertTrue($msg->isAdt());
+        self::assertFalse($msg->isOrm());
+        self::assertFalse($msg->isOru());
     }
 
     /**
@@ -493,4 +590,17 @@ public function repetition_separation_character_can_be_ignored(): void
         self::assertIsArray($patientIdentifierList);
         self::assertSame(['3', '0~4', '1'], $patientIdentifierList);
     }
+
+    /** @test */
+    public function a_message_can_be_saved_in_a_file(): void
+    {
+        $hl7File = __DIR__ . DIRECTORY_SEPARATOR . 'hl7_test_' . random_int(100, 1000) . '.hl7';
+        $message = new Message("MSH|^~\&|");
+        $message->toFile($hl7File);
+        self::assertFileExists($hl7File);
+        self::assertSame($message->toString(true), file_get_contents($hl7File));
+        if (file_exists($hl7File)) {
+            unlink($hl7File);
+        }
+    }
 }
diff --git a/tests/SegmentTest.php b/tests/SegmentTest.php
index b473842..ca8e46d 100644
--- a/tests/SegmentTest.php
+++ b/tests/SegmentTest.php
@@ -5,9 +5,31 @@
 namespace Aranyasen\HL7\Tests;
 
 use Aranyasen\HL7\Segment;
+use InvalidArgumentException;
 
 class SegmentTest extends TestCase
 {
+    /** @test */
+    public function it_throws_error_when_segment_name_has_less_than_3_characters(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        new Segment('XX');
+    }
+
+    /** @test */
+    public function it_throws_exception_when_a_blank_string_is_passed_as_segment_name(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        new Segment('');
+    }
+
+    /** @test */
+    public function it_throws_exception_when_segment_name_is_not_in_upper_case(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        new Segment('xxx');
+    }
+
     /** @test */
     public function field_at_a_given_nonzero_index_can_be_set(): void
     {