diff --git a/src/core/opamStd.ml b/src/core/opamStd.ml
index ffbf788f9cf..bac96349131 100644
--- a/src/core/opamStd.ml
+++ b/src/core/opamStd.ml
@@ -1086,6 +1086,8 @@ module OpamSys = struct
       | "cmd.exe" -> Some (Accept SH_cmd)
       | "env.exe" -> Some (Accept SH_sh)
       | name ->
+        if not (Filename.check_suffix name ".exe") then
+          Printf.printf "Unexpected: %S\n%!" name;
         Option.map
           (fun shell -> Accept shell)
           (shell_of_string (Filename.chop_suffix name ".exe"))