diff --git a/lib/libc/locale/utf8.c b/lib/libc/locale/utf8.c
index 02b1ccc936f..11e7825ae03 100644
--- a/lib/libc/locale/utf8.c
+++ b/lib/libc/locale/utf8.c
@@ -193,6 +193,13 @@ _UTF8_mbrtowc(wchar_t * __restrict pwc, const char * __restrict s, size_t n,
 		errno = EILSEQ;
 		return ((size_t)-1);
 	}
+	if (wch >= 0xd800 && wch <= 0xdfff) {
+		/*
+		 * Malformed input; invalid code points.
+		 */
+		errno = EILSEQ;
+		return ((size_t)-1);
+	}
 	if (pwc != NULL)
 		*pwc = wch;
 	us->want = 0;