diff --git a/lib/config/environment.js b/lib/config/environment.js
index a57fe0d..fc757cf 100644
--- a/lib/config/environment.js
+++ b/lib/config/environment.js
@@ -14,7 +14,7 @@ module.exports = {
   useSSL: toBooleanConfig(process.env.CMD_USESSL),
   hsts: {
     enable: toBooleanConfig(process.env.CMD_HSTS_ENABLE),
-    maxAgeSeconds: process.env.CMD_HSTS_MAX_AGE,
+    maxAgeSeconds: toIntegerConfig(process.env.CMD_HSTS_MAX_AGE),
     includeSubdomains: toBooleanConfig(process.env.CMD_HSTS_INCLUDE_SUBDOMAINS),
     preload: toBooleanConfig(process.env.CMD_HSTS_PRELOAD)
   },
diff --git a/lib/config/hackmdEnvironment.js b/lib/config/hackmdEnvironment.js
index e1c1156..bc20e58 100644
--- a/lib/config/hackmdEnvironment.js
+++ b/lib/config/hackmdEnvironment.js
@@ -10,7 +10,7 @@ module.exports = {
   useSSL: toBooleanConfig(process.env.HMD_USESSL),
   hsts: {
     enable: toBooleanConfig(process.env.HMD_HSTS_ENABLE),
-    maxAgeSeconds: process.env.HMD_HSTS_MAX_AGE,
+    maxAgeSeconds: toIntegerConfig(process.env.HMD_HSTS_MAX_AGE),
     includeSubdomains: toBooleanConfig(process.env.HMD_HSTS_INCLUDE_SUBDOMAINS),
     preload: toBooleanConfig(process.env.HMD_HSTS_PRELOAD)
   },