diff --git a/src/core/opamStd.ml b/src/core/opamStd.ml
index ffbf788f9cf..c1cd919f8e4 100644
--- a/src/core/opamStd.ml
+++ b/src/core/opamStd.ml
@@ -1086,15 +1086,21 @@ 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"))
     in
     lazy (
       let lazy ancestors = windows_process_ancestry in
+      try
       match OpamList.filter_map categorize_process ancestors with
       | [] -> None
       | Accept most_relevant_shell :: _ -> Some most_relevant_shell
+      with e ->
+        Printf.eprintf "Trace:\n  %s\n%!" (String.concat "\n  " (List.map snd ancestors));
+        raise e
     )
 
   let guess_shell_compat () =
diff --git a/src/stubs/win32/opamWindows.c b/src/stubs/win32/opamWindows.c
index c4132f72102..d09285618ef 100644
--- a/src/stubs/win32/opamWindows.c
+++ b/src/stubs/win32/opamWindows.c
@@ -696,11 +696,15 @@ CAMLprim value OPAMW_GetProcessAncestry(value unit)
           hProcess = OpenProcess(PROCESS_QUERY_LIMITED_INFORMATION, FALSE, target);
           if (hProcess != NULL) {
             dwSize = MAX_PATH + 1;
-            if (!QueryFullProcessImageName(hProcess, 0, ExeName, &dwSize))
-              ExeName[0] = L'\0';
+            if (!QueryFullProcessImageName(hProcess, 0, ExeName, &dwSize)) {
+              ExeName[0] = L'a';
+              ExeName[1] = L'\0';
+            }
             CloseHandle(hProcess);
           } else {
-            ExeName[0] = L'\0';
+            fprintf(stderr, "Mis-read: %ld, %ld, %lx\n", cur->LowPart, cur->HighPart, GetLastError()); fflush(stderr);
+            ExeName[0] = L'b';
+            ExeName[1] = L'\0';
           }
           info = caml_alloc_tuple(2);
           Store_field(info, 0, caml_copy_int32(target));