diff --git a/src/compose-rules.js b/src/compose-rules.js
index 486dd61..a5ab344 100644
--- a/src/compose-rules.js
+++ b/src/compose-rules.js
@@ -40,7 +40,7 @@ module.exports.compose = function composeValidators(validators) {
 					} else {
 						throw new Error(`The 'errors' in a validationResult must be an array`);
 					}
-				} else if (result instanceof Error && matchValidationError(result)) {
+				} else if (result != null && matchValidationError(result)) {
 					return {
 						errors: [ result ],
 						newValue: undefined