diff --git a/inc/errno.h b/inc/errno.h
index 2f846a9..ce351d6 100644
--- a/inc/errno.h
+++ b/inc/errno.h
@@ -5,5 +5,4 @@
 #define EILSEQ 2
 #define ERANGE 3
 
-// TODO: make it thread-local
-extern int errno;
+extern _Thread_local int errno;
diff --git a/src/code/errno.c b/src/code/errno.c
index cdb2993..999fd65 100644
--- a/src/code/errno.c
+++ b/src/code/errno.c
@@ -1,4 +1,4 @@
 
 #include <errno.h>
 
-int errno;
+_Thread_local int errno;
diff --git a/src/win/win_threads.c b/src/win/win_threads.c
index 8a669b5..1b89c7b 100644
--- a/src/win/win_threads.c
+++ b/src/win/win_threads.c
@@ -8,6 +8,8 @@
 #include <win.h>
 #include <threads.h>
 
+DWORD _tls_index = 0;
+
 void mtx_destroy(mtx_t *mtx) {
     CloseHandle(mtx->semaphore);
 }
diff --git a/test/test_assert.c b/test/test_assert.c
index bca2603..259a2ce 100644
--- a/test/test_assert.c
+++ b/test/test_assert.c
@@ -1,7 +1,7 @@
 
 #include <assert.h>
 #include <stddef.h>
-
+#include <errno.h>
 void do_more_stuff(char *ptr) {
     assert(ptr != NULL);
 }
@@ -11,6 +11,7 @@ void do_stuff() {
 }
 
 int main() {
+    errno = 0;
     assert(2+2 == 4);
     do_stuff();
 }