diff --git a/flask_sslify.py b/flask_sslify.py
index 19dfeb8..fcbf21e 100644
--- a/flask_sslify.py
+++ b/flask_sslify.py
@@ -54,5 +54,6 @@ class SSLify(object):
 
     def set_hsts_header(self, response):
         """Adds HSTS header to each response."""
-        response.headers.setdefault('Strict-Transport-Security', self.hsts_header)
+        if request.is_secure:
+            response.headers.setdefault('Strict-Transport-Security', self.hsts_header)
         return response