diff --git a/src/main/java/com/googlesource/gerrit/plugins/download/scheme/SshScheme.java b/src/main/java/com/googlesource/gerrit/plugins/download/scheme/SshScheme.java
index 8253b66..6e7cb15 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/download/scheme/SshScheme.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/download/scheme/SshScheme.java
@@ -17,6 +17,7 @@
 import static com.google.gerrit.reviewdb.client.AccountGeneralPreferences.DownloadScheme.DEFAULT_DOWNLOADS;
 import static com.google.gerrit.reviewdb.client.AccountGeneralPreferences.DownloadScheme.SSH;
 
+import com.google.common.base.Strings;
 import com.google.gerrit.common.Nullable;
 import com.google.gerrit.extensions.config.DownloadScheme;
 import com.google.gerrit.server.CurrentUser;
@@ -59,10 +60,14 @@
     if (!isEnabled() || !userProvider.get().isIdentifiedUser()) {
       return null;
     }
+    String username = userProvider.get().getUserName();
+    if (Strings.isNullOrEmpty(username)) {
+      return null;
+    }
 
     StringBuilder r = new StringBuilder();
     r.append("ssh://");
-    r.append(userProvider.get().getUserName());
+    r.append(username);
     r.append("@");
     r.append(ensureSlash(sshdAddress));
     r.append(project);
diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
index e4eae75..f5b697e 100644
--- a/src/main/resources/Documentation/about.md
+++ b/src/main/resources/Documentation/about.md
@@ -16,6 +16,8 @@
 * `HTTP`: Scheme for authenticated downloads via the HTTP protocol.
 
 * `SSH`: Scheme for authenticated downloads via the SSH protocol.
+<br />
+Requires that users have a username.
 
 * `REPO`: Scheme for downloading with the Repo tool.