diff --git a/python/obitools3/commands/ngsfilter.pyx b/python/obitools3/commands/ngsfilter.pyx
index 89ae5ba..bcbcd1e 100644
--- a/python/obitools3/commands/ngsfilter.pyx
+++ b/python/obitools3/commands/ngsfilter.pyx
@@ -42,7 +42,8 @@ def addOptions(parser):
                      metavar="<URI>",
                      type=str,
                      default=None,
-                     help="URI to the view containing the samples definition (with tags, primers, sample names,...)")
+                     help="URI to the view containing the samples definition (with tags, primers, sample names,...)"
+                          "Warning: primer lengths must be less than or equal to 32")
 
     group.add_argument('-R', '--reverse-reads',
                      action="store", dest="ngsfilter:reverse",