diff --git a/src/wp-admin/includes/taxonomy.php b/src/wp-admin/includes/taxonomy.php
index 124c0239a3..52cff7f23c 100644
--- a/src/wp-admin/includes/taxonomy.php
+++ b/src/wp-admin/includes/taxonomy.php
@@ -11,14 +11,17 @@
 //
 
 /**
- * {@internal Missing Short Description}}
+ * Check whether a category exists.
  *
  * @since 2.0.0
  *
- * @param int|string $cat_name
- * @return int
+ * @see term_exists()
+ *
+ * @param int|string $cat_name Category name.
+ * @param int        $parent   Optional. ID of parent term.
+ * @return mixed
  */
-function category_exists($cat_name, $parent = 0) {
+function category_exists( $cat_name, $parent = null ) {
 	$id = term_exists($cat_name, 'category', $parent);
 	if ( is_array($id) )
 		$id = $id['term_id'];
diff --git a/tests/phpunit/tests/term/categoryExists.php b/tests/phpunit/tests/term/categoryExists.php
new file mode 100644
index 0000000000..5f973d4437
--- /dev/null
+++ b/tests/phpunit/tests/term/categoryExists.php
@@ -0,0 +1,77 @@
+<?php
+
+class Tests_Term_CategoryExists extends WP_UnitTestCase {
+	/**
+	 * @ticket 30975
+	 */
+	public function test_category_exists_should_return_only_top_level_categories_when_parent_is_0() {
+		$c1 = $this->factory->category->create();
+		$c2 = $this->factory->category->create( array(
+			'name' => 'Foo',
+			'parent' => $c1,
+		) );
+		$c3 = $this->factory->category->create( array(
+			'name' => 'Foo',
+		) );
+
+		$found = category_exists( 'Foo', 0 );
+
+		$this->assertEquals( $found, $c3 );
+	}
+
+	/**
+	 * @ticket 30975
+	 */
+	public function test_category_exists_should_select_oldest_matching_category_when_no_parent_is_specified_1() {
+		// Foo child of c1 is created first.
+		$c1 = $this->factory->category->create();
+		$c2 = $this->factory->category->create( array(
+			'name' => 'Foo',
+			'parent' => $c1,
+		) );
+		$c3 = $this->factory->category->create( array(
+			'name' => 'Foo',
+		) );
+
+		$found = category_exists( 'Foo' );
+
+		$this->assertEquals( $found, $c2 );
+	}
+
+	/**
+	 * @ticket 30975
+	 */
+	public function test_category_exists_should_select_oldest_matching_category_when_no_parent_is_specified_2() {
+		// Top-level Foo is created first.
+		$c1 = $this->factory->category->create();
+		$c2 = $this->factory->category->create( array(
+			'name' => 'Foo',
+		) );
+		$c3 = $this->factory->category->create( array(
+			'name' => 'Foo',
+			'parent' => $c1,
+		) );
+
+		$found = category_exists( 'Foo' );
+
+		$this->assertEquals( $found, $c2 );
+	}
+
+	/**
+	 * @ticket 30975
+	 */
+	public function test_category_exists_should_respect_nonempty_parent() {
+		$c1 = $this->factory->category->create();
+		$c2 = $this->factory->category->create( array(
+			'name' => 'Foo',
+			'parent' => $c1,
+		) );
+		$c3 = $this->factory->category->create( array(
+			'name' => 'Foo',
+		) );
+
+		$found = category_exists( 'Foo', $c1 );
+
+		$this->assertEquals( $found, $c2 );
+	}
+}