diff --git a/custom-fields.php b/custom-fields.php
index 773560c..c1bc98d 100644
--- a/custom-fields.php
+++ b/custom-fields.php
@@ -47,6 +47,8 @@ function wpify_custom_fields(): CustomFields {
  * Only runs when the standalone plugin is deactivated,
  * not when the library is used as a Composer dependency.
  */
-register_deactivation_hook( __FILE__, function() {
-	wp_clear_scheduled_hook( 'wpifycf_cleanup_temp_files' );
-} );
+if ( function_exists( 'register_deactivation_hook' ) ) {
+	register_deactivation_hook( __FILE__, function () {
+		wp_clear_scheduled_hook( 'wpifycf_cleanup_temp_files' );
+	} );
+}