diff --git a/core/src/main/java/hudson/PluginManager.java b/core/src/main/java/hudson/PluginManager.java
index 7d9e73c7775227195a9deee9e913bf5af7be2c91..4867ba8faeb82082d6e2c57df668d9f88edfc685 100644
--- a/core/src/main/java/hudson/PluginManager.java
+++ b/core/src/main/java/hudson/PluginManager.java
@@ -732,7 +732,9 @@ public abstract class PluginManager extends AbstractModelObject implements OnMas
                     String pluginName = n.substring(0, index);
                     String siteName = n.substring(index + 1);
                     UpdateSite updateSite = Jenkins.getInstance().getUpdateCenter().getById(siteName);
-                    if (siteName != null) {
+                    if (updateSite == null) {
+                        throw new Failure("No such update center: " + siteName);
+                    } else {
                         UpdateSite.Plugin plugin = updateSite.getPlugin(pluginName);
                         if (plugin != null) {
                             if (p != null) {