diff --git a/hmp-commands.hx b/hmp-commands.hx
index 38e1eb7..372bef4 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -822,7 +822,7 @@ ETEXI
 
     {
         .name       = "snapshot_blkdev",
-        .args_type  = "device:s,snapshot_file:s?,format:s?",
+        .args_type  = "device:B,snapshot_file:s?,format:s?",
         .params     = "device [new-image-file] [format]",
         .help       = "initiates a live snapshot\n\t\t\t"
                       "of device. If a new image file is specified, the\n\t\t\t"