diff --git a/Changelog.md b/Changelog.md
index 72f34d2..b2be973 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -6,6 +6,10 @@
 - Fix JSON parsing of empty OCS results: the API serializes empty collections
   as `[]` instead of `{}`, which caused a `MismatchedInputException` when e.g.
   listing users or groups on an empty result (issue #112)
+- Fix file uploads (and other WebDAV writes) failing on servers running on a
+  non-standard port: preemptive authentication now uses the configured port, so
+  the server no longer issues an auth challenge that a streamed upload cannot
+  retry (issue #112)
 - Testing: integration tests can now auto-provision a throw-away Nextcloud
   server via Testcontainers when Docker is available, and are executed in CI
   (GitHub Actions)
diff --git a/src/main/java/org/aarboard/nextcloud/api/webdav/AWebdavHandler.java b/src/main/java/org/aarboard/nextcloud/api/webdav/AWebdavHandler.java
index b1f32b2..98c0f56 100644
--- a/src/main/java/org/aarboard/nextcloud/api/webdav/AWebdavHandler.java
+++ b/src/main/java/org/aarboard/nextcloud/api/webdav/AWebdavHandler.java
@@ -179,7 +179,13 @@ protected Sardine buildAuthSardine()
             Sardine sardine = SardineFactory.begin();
             sardine.setCredentials(this.serverConfig.getUserName(),
                     this.serverConfig.getAuthenticationConfig().getPassword());
-            sardine.enablePreemptiveAuthentication(this.serverConfig.getServerName());
+            // Pass the configured port so preemptive authentication also applies
+            // on non-standard ports (e.g. behind a reverse proxy). Without it the
+            // hostname-only overload assumes ports 80/443, the server then issues
+            // an auth challenge, and non-repeatable requests such as a streamed
+            // file upload (PUT) fail because they cannot be retried.
+            sardine.enablePreemptiveAuthentication(this.serverConfig.getServerName(),
+                    this.serverConfig.getPort(), this.serverConfig.getPort());
             return sardine;
         }
       return new SardineImpl(this.serverConfig.getAuthenticationConfig().getBearerToken());