diff --git src/wp-includes/class-wp-comment-query.php src/wp-includes/class-wp-comment-query.php
index eb6f094..71cd525 100644
--- src/wp-includes/class-wp-comment-query.php
+++ src/wp-includes/class-wp-comment-query.php
@@ -214,7 +214,7 @@ class WP_Comment_Query {
 	 *                                                   Default empty.
 	 *     @type int          $post_ID                   Currently unused.
 	 *     @type int          $post_id                   Limit results to those affiliated with a given post ID.
-	 *                                                   Default 0.
+	 *                                                   Default null.
 	 *     @type array        $post__in                  Array of post IDs to include affiliated comments for.
 	 *                                                   Default empty.
 	 *     @type array        $post__not_in              Array of post IDs to exclude affiliated comments for.
@@ -276,7 +276,7 @@ class WP_Comment_Query {
 			'post_author__in' => '',
 			'post_author__not_in' => '',
 			'post_ID' => '',
-			'post_id' => 0,
+			'post_id' => null,
 			'post__in' => '',
 			'post__not_in' => '',
 			'post_author' => '',
@@ -645,9 +645,8 @@ class WP_Comment_Query {
 			$fields = "$wpdb->comments.comment_ID";
 		}
 
-		$post_id = absint( $this->query_vars['post_id'] );
-		if ( ! empty( $post_id ) ) {
-			$this->sql_clauses['where']['post_id'] = $wpdb->prepare( 'comment_post_ID = %d', $post_id );
+		if ( strlen( $this->query_vars['post_id'] ) ) {
+			$this->sql_clauses['where']['post_id'] = $wpdb->prepare( 'comment_post_ID = %d', $this->query_vars['post_id'] );
 		}
 
 		// Parse comment IDs for an IN clause.
diff --git tests/phpunit/tests/comment/query.php tests/phpunit/tests/comment/query.php
index 6de468e..92dbcad 100644
--- tests/phpunit/tests/comment/query.php
+++ tests/phpunit/tests/comment/query.php
@@ -36,8 +36,12 @@ class Tests_Comment_Query extends WP_UnitTestCase {
 		$this->assertEqualSets( array( $c1, $c2, $c3, $c4, $c5 ), $found );
 	}
 
-	public function test_query_post_id_0() {
+	/**
+	 * @ticket 35090
+	 */
+	public function test_post_id_0_should_return_comments_with_no_parent() {
 		$c1 = self::factory()->comment->create( array( 'comment_post_ID' => self::$post_id, 'comment_approved' => '1' ) );
+		$c2 = self::factory()->comment->create( array( 'comment_post_ID' => 0, 'comment_approved' => '1' ) );
 
 		$q = new WP_Comment_Query();
 		$found = $q->query( array(
@@ -45,7 +49,71 @@ class Tests_Comment_Query extends WP_UnitTestCase {
 			'fields' => 'ids',
 		) );
 
-		$this->assertEqualSets( array( $c1 ), $found );
+		$this->assertEqualSets( array( $c2 ), $found );
+	}
+
+	/**
+	 * @ticket 35090
+	 */
+	public function test_post_id_string_0_should_return_comments_with_no_parent() {
+		$c1 = self::factory()->comment->create( array( 'comment_post_ID' => self::$post_id, 'comment_approved' => '1' ) );
+		$c2 = self::factory()->comment->create( array( 'comment_post_ID' => 0, 'comment_approved' => '1' ) );
+
+		$q = new WP_Comment_Query();
+		$found = $q->query( array(
+			'post_id' => '0',
+			'fields' => 'ids',
+		) );
+
+		$this->assertEqualSets( array( $c2 ), $found );
+	}
+
+	/**
+	 * @ticket 35090
+	 */
+	public function test_post_id_null_should_be_ignored() {
+		$c1 = self::factory()->comment->create( array( 'comment_post_ID' => self::$post_id, 'comment_approved' => '1' ) );
+		$c2 = self::factory()->comment->create( array( 'comment_post_ID' => 0, 'comment_approved' => '1' ) );
+
+		$q = new WP_Comment_Query();
+		$found = $q->query( array(
+			'post_id' => null,
+			'fields' => 'ids',
+		) );
+
+		$this->assertEqualSets( array( $c1, $c2 ), $found );
+	}
+
+	/**
+	 * @ticket 35090
+	 */
+	public function test_post_id_false_should_be_ignored() {
+		$c1 = self::factory()->comment->create( array( 'comment_post_ID' => self::$post_id, 'comment_approved' => '1' ) );
+		$c2 = self::factory()->comment->create( array( 'comment_post_ID' => 0, 'comment_approved' => '1' ) );
+
+		$q = new WP_Comment_Query();
+		$found = $q->query( array(
+			'post_id' => false,
+			'fields' => 'ids',
+		) );
+
+		$this->assertEqualSets( array( $c1, $c2 ), $found );
+	}
+
+	/**
+	 * @ticket 35090
+	 */
+	public function test_post_id_empty_string_should_be_ignored() {
+		$c1 = self::factory()->comment->create( array( 'comment_post_ID' => self::$post_id, 'comment_approved' => '1' ) );
+		$c2 = self::factory()->comment->create( array( 'comment_post_ID' => 0, 'comment_approved' => '1' ) );
+
+		$q = new WP_Comment_Query();
+		$found = $q->query( array(
+			'post_id' => '',
+			'fields' => 'ids',
+		) );
+
+		$this->assertEqualSets( array( $c1, $c2 ), $found );
 	}
 
 	/**