Index: src/org/openstreetmap/josm/gui/io/UploadDialogModel.java
===================================================================
--- src/org/openstreetmap/josm/gui/io/UploadDialogModel.java	(revision 18201)
+++ src/org/openstreetmap/josm/gui/io/UploadDialogModel.java	(working copy)
@@ -74,7 +74,7 @@
         String hashtags = String.join(";",
             Arrays.stream(comment.split("\\s", -1))
                 .map(s -> Utils.strip(s, ",;"))
-                .filter(s -> s.matches("#[a-zA-Z][-_a-zA-Z0-9]+"))
+                .filter(s -> s.matches("#[a-zA-Z0-9][-_a-zA-Z0-9]+"))
                 .collect(Collectors.toList()));
         return hashtags.isEmpty() ? null : hashtags;
     }