diff --git a/docs/backup-client.rst b/docs/backup-client.rst
index 1c2a98ed..e11c0142 100644
--- a/docs/backup-client.rst
+++ b/docs/backup-client.rst
@@ -169,6 +169,7 @@ the client. The format is:
 
     <archive-name>.<type>:<source-path>
 
+The ``archive-name`` must contain alphanumerics, hyphens and underscores only.
 Common types are ``.pxar`` for file archives and ``.img`` for block
 device images. To create a backup of a block device, run the following command:
 
diff --git a/pbs-client/src/backup_specification.rs b/pbs-client/src/backup_specification.rs
index b6fbc555..e44d23d9 100644
--- a/pbs-client/src/backup_specification.rs
+++ b/pbs-client/src/backup_specification.rs
@@ -7,10 +7,12 @@ const_regex! {
     BACKUPSPEC_REGEX = r"^([a-zA-Z0-9_-]+\.(pxar|img|conf|log)):(.+)$";
 }
 
-pub const BACKUP_SOURCE_SCHEMA: Schema =
-    StringSchema::new("Backup source specification ([<label>:<path>]).")
-        .format(&ApiStringFormat::Pattern(&BACKUPSPEC_REGEX))
-        .schema();
+pub const BACKUP_SOURCE_SCHEMA: Schema = StringSchema::new(
+    "Backup source specification ([<label>:<path>]), the specification \
+    'label' must contain alphanumerics, hyphens and underscores only.",
+)
+.format(&ApiStringFormat::Pattern(&BACKUPSPEC_REGEX))
+.schema();
 
 pub enum BackupSpecificationType {
     PXAR,
diff --git a/proxmox-backup-client/src/main.rs b/proxmox-backup-client/src/main.rs
index 589a097b..9077f7bd 100644
--- a/proxmox-backup-client/src/main.rs
+++ b/proxmox-backup-client/src/main.rs
@@ -628,7 +628,8 @@ fn spawn_catalog_upload(
        properties: {
            backupspec: {
                type: Array,
-               description: "List of backup source specifications ([<label.ext>:<path>] ...)",
+               description: "List of backup source specifications ([<label.ext>:<path>] ...), the \
+                    specifications 'label' must contain alphanumerics, hyphens and underscores only.",
                items: {
                    schema: BACKUP_SOURCE_SCHEMA,
                }