The Opengroup text says:
file
    A pathname of a text file to order. If no file operand is given, the standard input shall be used.

Keyword: text

a NUL byte has no place in a text file.


Index: tsort.c
===================================================================
RCS file: /build/data/openbsd/cvs/src/usr.bin/tsort/tsort.c,v
diff -u -p -r1.39 tsort.c
--- tsort.c	23 Jun 2026 08:27:37 -0000	1.39
+++ tsort.c	24 Jun 2026 11:16:39 -0000
@@ -312,19 +312,24 @@ read_pairs(FILE *f, struct ohash *h, int
 
 	while ((str = fgetln(f, &size)) != NULL) {
 		char *sentinel;
+		char *s;
 
 		sentinel = str + size;
+
+		for (s = str; s < sentinel; s++)
+			if (*s == '\0')
+				errx(1, "NUL byte detected in %s", name);
+
 		for (;;) {
 			char *e;
 
 			while (str < sentinel &&
-			    (isspace((unsigned char)*str) || *str == '\0'))
+			    (isspace((unsigned char)*str)))
 				str++;
 			if (str == sentinel)
 				break;
 			for (e = str;
-			    e < sentinel && !isspace((unsigned char)*e) &&
-			    *e != '\0'; e++)
+			    e < sentinel && !isspace((unsigned char)*e); e++)
 				continue;
 			if (toggle) {
 				a = node_lookup(h, str, e);