diff --git a/CHANGELOG.md b/CHANGELOG.md
index a81bbde..a8c966f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Changed
 - unit tests for PHPUnit 8
 
+### Fixed
+- error when formatting empty hook.
+
 ## 3.2 - 2019-01-30
 
 ### Added
diff --git a/src/Formatter/Hook_Formatter.php b/src/Formatter/Hook_Formatter.php
index 89ed11a..6051e2e 100644
--- a/src/Formatter/Hook_Formatter.php
+++ b/src/Formatter/Hook_Formatter.php
@@ -16,6 +16,7 @@ class Hook_Formatter {
 	 */
 	public function __construct() {
 
+		// TODO inject paths instead of reaching for global constants.
 		$this->truncate_paths = [
 			wp_normalize_path( WP_CONTENT_DIR ),
 			wp_normalize_path( ABSPATH ),
@@ -35,6 +36,11 @@ class Hook_Formatter {
 		if ( $hook instanceof \WP_Hook ) {
 			$hook = $hook->callbacks;
 		}
+
+		if ( empty( $hook ) ) {
+			return $callbacks;
+		}
+
 		ksort( $hook );
 
 		$functions = array_merge( ...$hook );
diff --git a/tests/Formatter/HookTest.php b/tests/Formatter/HookTest.php
index 246781b..7843ff0 100644
--- a/tests/Formatter/HookTest.php
+++ b/tests/Formatter/HookTest.php
@@ -55,6 +55,8 @@ class HookTest extends LapsTestCase {
 		$this->assertEquals( 'class::method', $result[1] );
 		$this->assertStringStartsWith( 'closure from', $result[2] );
 		$this->assertStringStartsWith( 'anonymous class from', $result[3] );
+
+		$this->assertEquals( [], $formatter->format( [] ) );
 	}
 }