diff --git a/src/cloudron.js b/src/cloudron.js
index b2f5b9f02..38dd0a9c5 100644
--- a/src/cloudron.js
+++ b/src/cloudron.js
@@ -443,7 +443,7 @@ function doUpgrade(boxUpdateInfo, callback) {
     assert(boxUpdateInfo !== null && typeof boxUpdateInfo === 'object');
 
     function upgradeError(e) {
-        progress.set(progress.UPDATE, 100, e.message);
+        progress.set(progress.UPDATE, -1, e.message);
         callback(e);
     }
 
@@ -472,7 +472,7 @@ function doUpdate(boxUpdateInfo, callback) {
     assert(boxUpdateInfo && typeof boxUpdateInfo === 'object');
 
     function updateError(e) {
-        progress.set(progress.UPDATE, 100, e.message);
+        progress.set(progress.UPDATE, -1, e.message);
         callback(e);
     }
 
diff --git a/src/progress.js b/src/progress.js
index 6e3be7d9e..042b5bdfb 100644
--- a/src/progress.js
+++ b/src/progress.js
@@ -21,6 +21,7 @@ var progress = {
     backup: null
 };
 
+// We use -1 for percentage to indicate errors
 function set(tag, percent, message) {
     assert(tag === exports.UPDATE || tag === exports.BACKUP);
     assert.strictEqual(typeof percent, 'number');