diff --git a/changelog.d/6421.bugfix b/changelog.d/6421.bugfix
new file mode 100644
index 000000000..7969f7f71
--- /dev/null
+++ b/changelog.d/6421.bugfix
@@ -0,0 +1 @@
+Fix startup error when http proxy is defined.
diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
index 15c15a12f..87343d9db 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -77,8 +77,8 @@ class PreviewUrlResource(DirectServeResource):
             treq_args={"browser_like_redirects": True},
             ip_whitelist=hs.config.url_preview_ip_range_whitelist,
             ip_blacklist=hs.config.url_preview_ip_range_blacklist,
-            http_proxy=os.getenv("http_proxy"),
-            https_proxy=os.getenv("HTTPS_PROXY"),
+            http_proxy=os.getenvb(b"http_proxy"),
+            https_proxy=os.getenvb(b"HTTPS_PROXY"),
         )
         self.media_repo = media_repo
         self.primary_base_path = media_repo.primary_base_path
diff --git a/synapse/server.py b/synapse/server.py
index 90c3b072e..be9af7f98 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -318,8 +318,8 @@ class HomeServer(object):
     def build_proxied_http_client(self):
         return SimpleHttpClient(
             self,
-            http_proxy=os.getenv("http_proxy"),
-            https_proxy=os.getenv("HTTPS_PROXY"),
+            http_proxy=os.getenvb(b"http_proxy"),
+            https_proxy=os.getenvb(b"HTTPS_PROXY"),
         )
 
     def build_room_creation_handler(self):