diff --git a/src/Collection/Iterator/SingletonIterator.php b/src/Collection/Iterator/SingletonIterator.php
index 8ad88ba..eed6ff3 100644
--- a/src/Collection/Iterator/SingletonIterator.php
+++ b/src/Collection/Iterator/SingletonIterator.php
@@ -64,4 +64,9 @@ public function rewind(): void
     {
         $this->hasNext = true;
     }
+
+    public function key(): mixed
+    {
+        return 0;
+    }
 }
diff --git a/src/Control/Either.php b/src/Control/Either.php
index e79a327..f320f04 100644
--- a/src/Control/Either.php
+++ b/src/Control/Either.php
@@ -17,10 +17,6 @@
  */
 abstract class Either extends Value
 {
-    private function __construct()
-    {
-    }
-
     /**
      * @template L1
      *
diff --git a/tests/Collection/GenericListTest.php b/tests/Collection/GenericListTest.php
index c5fcdc7..6c7fe67 100644
--- a/tests/Collection/GenericListTest.php
+++ b/tests/Collection/GenericListTest.php
@@ -256,4 +256,18 @@ public function testIndexOfOnEmptyList(): void
     {
         self::assertSame(-1, GenericList::empty()->indexOf('a'));
     }
+
+    public function testHeadOfEmptyList(): void
+    {
+        $this->expectException(\RuntimeException::class);
+
+        GenericList::empty()->head();
+    }
+
+    public function testTailOfEmptyList(): void
+    {
+        $this->expectException(\RuntimeException::class);
+
+        GenericList::empty()->tail();
+    }
 }
diff --git a/tests/Collection/Iterator/SingletonIteratorTest.php b/tests/Collection/Iterator/SingletonIteratorTest.php
new file mode 100644
index 0000000..b137cf4
--- /dev/null
+++ b/tests/Collection/Iterator/SingletonIteratorTest.php
@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Munus\Tests\Collection\Iterator;
+
+use Munus\Collection\Iterator\SingletonIterator;
+use Munus\Exception\NoSuchElementException;
+use PHPUnit\Framework\TestCase;
+
+final class SingletonIteratorTest extends TestCase
+{
+    public function testCurrentIfThereIsNoNext(): void
+    {
+        $iterator = new SingletonIterator('a');
+        $iterator->next();
+
+        $this->expectException(NoSuchElementException::class);
+
+        $iterator->current();
+    }
+
+    public function testRewind(): void
+    {
+        $iterator = new SingletonIterator('a');
+        self::assertSame('a', $iterator->next());
+        self::assertFalse($iterator->hasNext());
+
+        $iterator->rewind();
+
+        self::assertTrue($iterator->hasNext());
+        self::assertTrue($iterator->valid());
+        self::assertSame(0, $iterator->key());
+        self::assertSame('a', $iterator->next());
+        self::assertFalse($iterator->hasNext());
+    }
+}
diff --git a/tests/Collection/IteratorTest.php b/tests/Collection/IteratorTest.php
index 7cb244b..1db2918 100644
--- a/tests/Collection/IteratorTest.php
+++ b/tests/Collection/IteratorTest.php
@@ -7,6 +7,7 @@
 use Munus\Collection\Iterator;
 use Munus\Collection\Iterator\CompositeIterator;
 use Munus\Collection\Map;
+use Munus\Collection\Set;
 use Munus\Exception\NoSuchElementException;
 use Munus\Tuple;
 use PHPUnit\Framework\TestCase;
@@ -80,4 +81,22 @@ public function testReduce(): void
 
         self::assertEquals(6, $iterator->reduce(fn (int $a, int $b) => $a + $b));
     }
+
+    public function testReduceOnEmpty(): void
+    {
+        $this->expectException(NoSuchElementException::class);
+
+        Iterator::empty()->reduce(fn (int $a, int $b) => $a + $b);
+    }
+
+    public function testIteratorKey(): void
+    {
+        $iterator = new Iterator(Set::of(1, 2, 3));
+
+        self::assertSame(0, $iterator->key());
+        $iterator->next();
+        self::assertSame(1, $iterator->key());
+        $iterator->next();
+        self::assertSame(2, $iterator->key());
+    }
 }
diff --git a/tests/Collection/StreamTest.php b/tests/Collection/StreamTest.php
index 4b78391..885a640 100644
--- a/tests/Collection/StreamTest.php
+++ b/tests/Collection/StreamTest.php
@@ -92,9 +92,10 @@ public function testStreamFilterNot(): void
 
     public function testStreamLength(): void
     {
-        self::assertEquals(5, Stream::from(0)->take(5)->length());
-        self::assertEquals(5, Stream::range(6, 10)->length());
-        self::assertEquals(5, Stream::ofAll(str_split('Munus'))->length());
+        self::assertSame(5, Stream::from(0)->take(5)->length());
+        self::assertSame(5, Stream::range(6, 10)->length());
+        self::assertSame(5, Stream::ofAll(str_split('Munus'))->length());
+        self::assertSame(0, Stream::empty()->length());
     }
 
     public function testStreamContains(): void
@@ -263,6 +264,13 @@ public function testStreamPrependAll(): void
         self::assertTrue(Stream::of('a', 'b', 'c', 'd', 'e')->equals(Stream::of('e')->prependAll(Stream::of('a', 'b', 'c', 'd'))));
     }
 
+    public function testStreamPrependAllEmptyStream(): void
+    {
+        $stream = Stream::of(1, 2, 3);
+
+        self::assertSame($stream, $stream->prependAll(Stream::empty()));
+    }
+
     public function testStreamToArray(): void
     {
         self::assertEquals([1, 2, 3, 4, 5], Stream::range(1, 5)->toArray());
@@ -306,4 +314,18 @@ public function testIndexOfOnEmptyStream(): void
     {
         self::assertSame(-1, Stream::empty()->indexOf('a'));
     }
+
+    public function testHeadOfEmptyStream(): void
+    {
+        $this->expectException(\RuntimeException::class);
+
+        Stream::empty()->head();
+    }
+
+    public function testTailOfEmptyStream(): void
+    {
+        $this->expectException(\RuntimeException::class);
+
+        Stream::empty()->tail();
+    }
 }
diff --git a/tests/Control/EitherTest.php b/tests/Control/EitherTest.php
index ee1cd87..827a920 100644
--- a/tests/Control/EitherTest.php
+++ b/tests/Control/EitherTest.php
@@ -119,4 +119,20 @@ public function testToArray(): void
         self::assertEquals(['a'], Either::right('a')->toArray());
         self::assertEquals([], Either::left('a')->toArray());
     }
+
+    public function testEitherIteratorLeft(): void
+    {
+        $iterator = Either::left('left')->iterator();
+
+        self::assertFalse($iterator->hasNext());
+    }
+
+    public function testEitherIteratorRight(): void
+    {
+        $iterator = Either::right('right')->iterator();
+
+        self::assertTrue($iterator->hasNext());
+        self::assertSame('right', $iterator->next());
+        self::assertFalse($iterator->hasNext());
+    }
 }
diff --git a/tests/Control/OptionTest.php b/tests/Control/OptionTest.php
index 4d5c1fe..3cd6e9a 100644
--- a/tests/Control/OptionTest.php
+++ b/tests/Control/OptionTest.php
@@ -188,4 +188,11 @@ public function testOptionIfPresent(): void
         Option::none()->ifPresent(fn ($v) => throw new \RuntimeException('impossible is nothing'));
         Option::of('a')->ifPresent(fn ($v) => self::assertSame('a', $v));
     }
+
+    public function testOptionOfNoneGet(): void
+    {
+        $this->expectException(\RuntimeException::class);
+
+        Option::none()->get();
+    }
 }
diff --git a/tests/Control/TryToTest.php b/tests/Control/TryToTest.php
index c667d7b..b6ae4d9 100644
--- a/tests/Control/TryToTest.php
+++ b/tests/Control/TryToTest.php
@@ -271,4 +271,34 @@ public function testSuccessPeek(): void
         self::assertSame($try, $try->peek(function ($value) use (&$check) {$check = $value; }));
         self::assertEquals('munus', $check);
     }
+
+    public function testTryToIteratorSuccess(): void
+    {
+        $iterator = TryTo::run(function () {return 'munus'; })->iterator();
+
+        self::assertTrue($iterator->hasNext());
+        self::assertSame('munus', $iterator->next());
+        self::assertFalse($iterator->hasNext());
+    }
+
+    public function testTryToIteratorFailure(): void
+    {
+        $iterator = TryTo::run(fn () => throw new \RuntimeException())->iterator();
+
+        self::assertFalse($iterator->hasNext());
+    }
+
+    public function testTryToGetCauseOnSuccess(): void
+    {
+        $this->expectException(\BadMethodCallException::class);
+
+        TryTo::run(function () {return 'munus'; })->getCause();
+    }
+
+    public function testTryToGetOnFailure(): void
+    {
+        $this->expectException(\BadMethodCallException::class);
+
+        TryTo::run(fn () => throw new \RuntimeException())->get();
+    }
 }
diff --git a/tests/LazyTest.php b/tests/LazyTest.php
index 51fb5fa..937a372 100644
--- a/tests/LazyTest.php
+++ b/tests/LazyTest.php
@@ -28,6 +28,14 @@ public function testEvaluation(): void
         self::assertTrue($lazy->isEvaluated());
     }
 
+    public function testInvoke(): void
+    {
+        $lazy = Lazy::of(function () {return 'Munus is awesome'; });
+
+        self::assertEquals('Munus is awesome', $lazy());
+        self::assertTrue($lazy->isEvaluated());
+    }
+
     public function testEvaluationOnlyOnce(): void
     {
         $lazy = Lazy::of(function (): int {return random_int(1, 1000); });
@@ -81,4 +89,13 @@ public function testLazyPeek(): void
         self::assertEquals('munus', $check);
         self::assertTrue($lazy->isEvaluated());
     }
+
+    public function testLazyIterator(): void
+    {
+        $iterator = Lazy::of(function () {return 'munus'; })->iterator();
+
+        self::assertTrue($iterator->hasNext());
+        self::assertSame('munus', $iterator->next());
+        self::assertFalse($iterator->hasNext());
+    }
 }