the C standard doesn't prohibit a zero argc, from C89 5.1.2.2:

> * The value of argc shall be nonnegative.
> * argv[argc] shall be a null pointer.
> * If the value of argc is greater than zero, the string pointed to by
>   argv[0] represents the program name (...)

this situation can be triggered by calling exec with NULL argv[0] and
tested by applying the following patch to test.c:

	     if (argc > 1) {
	+        execv(argv[0], (char *[]){NULL});
	+    } else if (argc == 0) {
	         return manual_test(argc, argv);
	     } else {
	         return testsuite();

not sure how likely it is for such situation to occurs in practice,
but deal with it in any case by setting optind to 0 if argv[0] happens
to be null.
---
 optparse.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/optparse.h b/optparse.h
index f96184a..8d6c0a9 100644
--- a/optparse.h
+++ b/optparse.h
@@ -143,7 +143,7 @@ optparse_init(struct optparse *options, char **argv)
 {
     options->argv = argv;
     options->permute = 1;
-    options->optind = 1;
+    options->optind = argv[0] != 0;
     options->subopt = 0;
     options->optarg = 0;
     options->errmsg[0] = '\0';
-- 
2.35.1

Thanks, NRK! Applied and pushed. I thought I had fixed this already, but 
that was a different project.