diff --git a/gnu/packages/c.scm b/gnu/packages/c.scm
index c49c311fd0..7a7408b4d7 100644
--- a/gnu/packages/c.scm
+++ b/gnu/packages/c.scm
@@ -184,6 +184,8 @@ (define-public libbytesize
 (define-public udunits
   (package
     (name "udunits")
+    ;; Four-part version numbers are development snapshots, not releases.  See
+    ;; <https://github.com/Unidata/UDUNITS-2/issues/99#issuecomment-732323472>.
     (version "2.2.26")
     (source (origin
               (method url-fetch)