diff --git a/index.js b/index.js
index d8e878b..37b03a1 100644
--- a/index.js
+++ b/index.js
@@ -54,18 +54,23 @@ module.exports = function hasShape(rules) {
 			let errors = [];
 			let newObject = {};
 
-			for (let [ key, rule ] of Object.entries(rules)) {
-				let value = object[key];
+			for (let [ key, value ] of Object.entries(object)) {
+				let rule = rules[key];
 
-				let { errors: keyErrors, newValue } = applyValidators(value, rule);
+				if (rule != null) {
+					let { errors: keyErrors, newValue } = applyValidators(value, rule);
 
-				let annotatedErrors = annotateErrors({
-					pathSegments: [ key ],
-					errors: keyErrors
-				});
+					let annotatedErrors = annotateErrors({
+						pathSegments: [ key ],
+						errors: keyErrors
+					});
 
-				errors.push(... annotatedErrors);
-				newObject[key] = newValue;
+					errors.push(... annotatedErrors);
+					newObject[key] = newValue;
+				} else {
+					// Extraneous property
+					newObject[key] = value;
+				}
 			}
 
 			return validationResult({