diff --git a/lib/server.js b/lib/server.js
index 90ea981..85cdb55 100644
--- a/lib/server.js
+++ b/lib/server.js
@@ -212,11 +212,11 @@ function Server(contentDirectory, lambdaApi, logger) {
 
         const responseHeaders = cloneDeep(response.headers || {});
         Object.keys(responseHeaders).filter(h => h.match(/location/i)).forEach(header => {
-          responseHeaders[header] = responseHeaders[header].replace(/^https?:\/\/(localhost:\d{2,5})\/(.*)$/g, 'http://$1/api/$2');
+          responseHeaders[header] = `${responseHeaders[header]}`.replace(/^https?:\/\/(localhost:\d{2,5})\/(.*)$/g, 'http://$1/api/$2');
         });
         const multiValueHeaders = cloneDeep(response.multiValueHeaders || {});
         Object.keys(multiValueHeaders).filter(h => multiValueHeaders[h]).forEach(h => {
-          responseHeaders[h] = multiValueHeaders[h].filter(v => v).map(v => v.replace(/^https?:\/\/(localhost:\d{2,5})\/(.*)$/g, 'http://$1/api/$2'));
+          responseHeaders[h] = multiValueHeaders[h].filter(v => v).map(v => `${v}`.replace(/^https?:\/\/(localhost:\d{2,5})\/(.*)$/g, 'http://$1/api/$2'));
         });
 
         if (Array.isArray(responseHeaders['Content-Type'])) {