diff --git a/usr/src/lib/libc/port/locale/mbtowc.c b/usr/src/lib/libc/port/locale/mbtowc.c
index 90dc2e6..ac71fc2 100644
--- a/usr/src/lib/libc/port/locale/mbtowc.c
+++ b/usr/src/lib/libc/port/locale/mbtowc.c
@@ -45,9 +45,15 @@ mbtowc_l(wchar_t *_RESTRICT_KYWD pwc, const char *_RESTRICT_KYWD s, size_t n,
 		return (0);
 	}
 	rval = mbrtowc_l(pwc, s, n, &mbs, loc);
-	if (rval == (size_t)-1 || rval == (size_t)-2)
+	switch (rval) {
+	case (size_t)-2:
+		errno = EILSEQ;
+		/* FALLTHROUGH */
+	case (size_t)-1:
 		return (-1);
-	return ((int)rval);
+	default:
+		return ((int)rval);
+	}
 }
 
 int