diff --git a/libc/bionic/pthread.c b/libc/bionic/pthread.c
index 8171aac19..c1a6a8aaa 100644
--- a/libc/bionic/pthread.c
+++ b/libc/bionic/pthread.c
@@ -1688,7 +1688,17 @@ extern int __rt_sigprocmask(int, const sigset_t *, sigset_t *, size_t);
 
 int pthread_sigmask(int how, const sigset_t *set, sigset_t *oset)
 {
-    return __rt_sigprocmask(how, set, oset, _NSIG / 8);
+    /* pthread_sigmask must return the error code, but the syscall
+     * will set errno instead and return 0/-1
+     */
+    int ret, old_errno = errno;
+
+    ret = __rt_sigprocmask(how, set, oset, _NSIG / 8);
+    if (ret < 0)
+        ret = errno;
+
+    errno = old_errno;
+    return ret;
 }
 
 
diff --git a/libc/docs/CHANGES.TXT b/libc/docs/CHANGES.TXT
index e818df3ed..47289f615 100644
--- a/libc/docs/CHANGES.TXT
+++ b/libc/docs/CHANGES.TXT
@@ -64,6 +64,10 @@ Differences between current and Android 2.1:
   in /etc/resolv.conf. (resolv.conf is already ignored, so the latter
   is a no-op for actual functionality.)
 
+- fix pthread_sigmask() to properly return an error code without touching
+  errno. Previous implementation returned -1 on error, setting errno, which
+  is not Posix compliant.
+
 -------------------------------------------------------------------------------
 Differences between Android 2.1 and 2.0.1: