Addressed some missed code coverage

tests/SerializeListTest.php

@@ -2,8 +2,10 @@
 
 namespace gapple\Tests\StructuredFields;
 
+use gapple\StructuredFields\Item;
 use gapple\StructuredFields\OuterList;
 use gapple\StructuredFields\Parameters;
+use gapple\StructuredFields\SerializeException;
 use gapple\StructuredFields\Serializer;
 use PHPUnit\Framework\TestCase;
 
@@ -12,7 +14,7 @@ class SerializeListTest extends TestCase
     /**
      * A list with bare tuples.
      */
-    public function testUntypedList(): void
+    public function testUntypedListItems(): void
     {
         $itemParam = new \stdClass();
         $itemParam->item_param = 32;
@@ -36,4 +38,31 @@ public function testUntypedList(): void
             $serialized
         );
     }
+
+    public function testInvalidListItem(): void
+    {
+        $this->expectException(SerializeException::class);
+
+        $list = [
+            new Item('test'),
+            'test', // Lists can't contain bare items.
+        ];
+
+        Serializer::serializeList($list); // @phpstan-ignore-line
+    }
+
+    public function testIterable(): void
+    {
+        $list = new \ArrayObject([
+            new Item('test'),
+            new Item(42),
+        ]);
+
+        $serialized = Serializer::serializeList($list);
+
+        $this->assertEquals(
+            '"test", 42',
+            $serialized
+        );
+    }
 }

tests/TupleTraitTest.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace gapple\Tests\StructuredFields;
+
+use gapple\StructuredFields\Item;
+use PHPUnit\Framework\TestCase;
+
+class TupleTraitTest extends TestCase
+{
+    public function testSetInvalidParameterValue(): void
+    {
+        $this->expectException(\InvalidArgumentException::class);
+
+        $item = new Item('test');
+
+        $item[1] = 'test';
+    }
+}