diff --git a/include/stdlib.h b/include/stdlib.h
index 592a387b..f388a194 100644
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -46,7 +46,7 @@ void exit (int status);
 void free (void *ptr);
 char *getenv (char const *s);
 int setenv (char const *s, char const *v, int overwrite_p);
-void unsetenv (char const *name);
+int unsetenv (char const *name);
 void *malloc (size_t);
 void qsort (void *base, size_t nmemb, size_t size, int (*compar) (void const *, void const *));
 int rand (void);
diff --git a/lib/posix/unsetenv.c b/lib/posix/unsetenv.c
index 726f6267..b81fe324 100644
--- a/lib/posix/unsetenv.c
+++ b/lib/posix/unsetenv.c
@@ -1,6 +1,6 @@
 /* -*-comment-start: "//";comment-end:""-*-
  * GNU Mes --- Maxwell Equations of Software
- * Copyright © 2018 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ * Copyright © 2018,2022 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
  *
  * This file is part of GNU Mes.
  *
@@ -22,7 +22,7 @@
 #include <string.h>
 #include <unistd.h>
 
-void
+int
 unsetenv (char const *name)
 {
   int length = strlen (name);
@@ -38,4 +38,5 @@ unsetenv (char const *name)
         }
       p++;
     }
+  return 0;
 }