diff --git a/src/afd.c b/src/afd.c
index 1b6e8fe..8c32333 100644
--- a/src/afd.c
+++ b/src/afd.c
@@ -71,7 +71,7 @@ int afd_global_init(void) {
   /* Load the winsock catalog. */
   infos_count = ws_get_protocol_catalog(&infos);
   if (infos_count < 0)
-    return_error(-1);
+    return_map_error(-1);
 
   /* Find a WSAPROTOCOL_INFOW structure that we can use to create an MSAFD
    * socket. Preferentially we pick a UDP socket, otherwise try TCP or any
@@ -90,7 +90,7 @@ int afd_global_init(void) {
       break;
 
     free(infos);
-    return_error(-1, WSAENETDOWN); /* No suitable protocol found. */
+    return_set_error(-1, WSAENETDOWN); /* No suitable protocol found. */
   }
 
   /* Copy found protocol information from the catalog to a static buffer. */
@@ -110,7 +110,7 @@ int afd_create_driver_socket(HANDLE iocp, SOCKET* driver_socket_out) {
                       0,
                       WSA_FLAG_OVERLAPPED);
   if (socket == INVALID_SOCKET)
-    return_error(-1);
+    return_map_error(-1);
 
   /* TODO: use WSA_FLAG_NOINHERIT on Windows versions that support it. */
   if (!SetHandleInformation((HANDLE) socket, HANDLE_FLAG_INHERIT, 0))
@@ -125,7 +125,7 @@ int afd_create_driver_socket(HANDLE iocp, SOCKET* driver_socket_out) {
 error:;
   DWORD error = GetLastError();
   closesocket(socket);
-  return_error(-1, error);
+  return_set_error(-1, error);
 }
 
 int afd_poll(SOCKET driver_socket,
@@ -155,7 +155,7 @@ int afd_poll(SOCKET driver_socket,
     iosb_ptr = &iosb;
     event = CreateEventW(NULL, FALSE, FALSE, NULL);
     if (event == NULL)
-      return_error(-1);
+      return_map_error(-1);
     apc_context = NULL;
   }
 
@@ -180,7 +180,7 @@ int afd_poll(SOCKET driver_socket,
       if (r == WAIT_FAILED) {
         DWORD error = GetLastError();
         CloseHandle(event);
-        return_error(-1, error);
+        return_set_error(-1, error);
       }
 
       status = iosb_ptr->Status;
@@ -192,7 +192,7 @@ int afd_poll(SOCKET driver_socket,
   if (status == STATUS_SUCCESS)
     return 0;
   else if (status == STATUS_PENDING)
-    return_error(-1, ERROR_IO_PENDING);
+    return_set_error(-1, ERROR_IO_PENDING);
   else
-    return_error(-1, RtlNtStatusToDosError(status));
+    return_set_error(-1, RtlNtStatusToDosError(status));
 }
diff --git a/src/api.c b/src/api.c
index 76f95a5..ad9b1f1 100644
--- a/src/api.c
+++ b/src/api.c
@@ -36,7 +36,7 @@ static HANDLE _epoll_create(void) {
                   (uintptr_t) ephnd) < 0) {
     /* This should never happen. */
     ep_port_delete(port_info);
-    return_error(NULL, ERROR_ALREADY_EXISTS);
+    return_set_error(NULL, ERROR_ALREADY_EXISTS);
   }
 
   return ephnd;
@@ -44,14 +44,14 @@ static HANDLE _epoll_create(void) {
 
 HANDLE epoll_create(int size) {
   if (size <= 0)
-    return_error(NULL, ERROR_INVALID_PARAMETER);
+    return_set_error(NULL, ERROR_INVALID_PARAMETER);
 
   return _epoll_create();
 }
 
 HANDLE epoll_create1(int flags) {
   if (flags != 0)
-    return_error(NULL, ERROR_INVALID_PARAMETER);
+    return_set_error(NULL, ERROR_INVALID_PARAMETER);
 
   return _epoll_create();
 }
@@ -122,7 +122,7 @@ int epoll_wait(HANDLE ephnd,
   int num_events;
 
   if (maxevents <= 0)
-    return_error(-1, ERROR_INVALID_PARAMETER);
+    return_set_error(-1, ERROR_INVALID_PARAMETER);
 
   if (init() < 0)
     return -1;
diff --git a/src/error.c b/src/error.c
index afb7af1..958061f 100644
--- a/src/error.c
+++ b/src/error.c
@@ -117,11 +117,12 @@ static errno_t _err_map_win_error_to_errno(DWORD error) {
   return EINVAL;
 }
 
+void err_map_win_error(void) {
+  errno = _err_map_win_error_to_errno(GetLastError());
+}
+
 void err_set_win_error(DWORD error) {
-  if (error == 0)
-    error = GetLastError();
-  else
-    SetLastError(error);
+  SetLastError(error);
   errno = _err_map_win_error_to_errno(error);
 }
 
@@ -131,10 +132,10 @@ int err_check_handle(HANDLE handle) {
   /* GetHandleInformation() succeeds when passed INVALID_HANDLE_VALUE, so check
    * for this condition explicitly. */
   if (handle == INVALID_HANDLE_VALUE)
-    return_error(-1, ERROR_INVALID_HANDLE);
+    return_set_error(-1, ERROR_INVALID_HANDLE);
 
   if (!GetHandleInformation(handle, &flags))
-    return_error(-1);
+    return_map_error(-1);
 
   return 0;
 }
diff --git a/src/error.h b/src/error.h
index dd1f290..6efc258 100644
--- a/src/error.h
+++ b/src/error.h
@@ -4,14 +4,19 @@
 #include "internal.h"
 #include "win.h"
 
-#define _return_error_helper(error, value) \
-  do {                                     \
-    err_set_win_error(error);              \
-    return (value);                        \
+#define return_map_error(value) \
+  do {                          \
+    err_map_win_error();        \
+    return (value);             \
   } while (0)
 
-#define return_error(value, ...) _return_error_helper(__VA_ARGS__ + 0, value)
+#define return_set_error(value, error) \
+  do {                                 \
+    err_set_win_error(error);          \
+    return (value);                    \
+  } while (0)
 
+WEPOLL_INTERNAL void err_map_win_error(void);
 WEPOLL_INTERNAL void err_set_win_error(DWORD error);
 WEPOLL_INTERNAL int err_check_handle(HANDLE handle);
 
diff --git a/src/poll-group.c b/src/poll-group.c
index 9a40e11..d1b379c 100644
--- a/src/poll-group.c
+++ b/src/poll-group.c
@@ -20,7 +20,7 @@ typedef struct poll_group {
 static poll_group_t* _poll_group_new(ep_port_t* port_info) {
   poll_group_t* poll_group = malloc(sizeof *poll_group);
   if (poll_group == NULL)
-    return_error(NULL, ERROR_NOT_ENOUGH_MEMORY);
+    return_set_error(NULL, ERROR_NOT_ENOUGH_MEMORY);
 
   memset(poll_group, 0, sizeof *poll_group);
 
diff --git a/src/port.c b/src/port.c
index 7d42a87..32d2db7 100644
--- a/src/port.c
+++ b/src/port.c
@@ -19,7 +19,7 @@
 static ep_port_t* _ep_port_alloc(void) {
   ep_port_t* port_info = malloc(sizeof *port_info);
   if (port_info == NULL)
-    return_error(NULL, ERROR_NOT_ENOUGH_MEMORY);
+    return_set_error(NULL, ERROR_NOT_ENOUGH_MEMORY);
 
   return port_info;
 }
@@ -32,7 +32,7 @@ static void _ep_port_free(ep_port_t* port) {
 static HANDLE _ep_port_create_iocp(void) {
   HANDLE iocp = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);
   if (iocp == NULL)
-    return_error(NULL);
+    return_map_error(NULL);
 
   return iocp;
 }
@@ -73,7 +73,7 @@ static int _ep_port_close_iocp(ep_port_t* port_info) {
   port_info->iocp = NULL;
 
   if (!CloseHandle(iocp))
-    return_error(-1);
+    return_map_error(-1);
 
   return 0;
 }
@@ -185,7 +185,7 @@ static int _ep_port_poll(ep_port_t* port_info,
   port_info->active_poll_count--;
 
   if (!r)
-    return_error(-1);
+    return_map_error(-1);
 
   return _ep_port_feed_events(
       port_info, epoll_events, iocp_events, completion_count);
@@ -203,7 +203,7 @@ int ep_port_wait(ep_port_t* port_info,
 
   /* Check whether `maxevents` is in range. */
   if (maxevents <= 0)
-    return_error(-1, ERROR_INVALID_PARAMETER);
+    return_set_error(-1, ERROR_INVALID_PARAMETER);
 
   /* Decide whether the IOCP completion list can live on the stack, or allocate
    * memory for it on the heap. */
@@ -322,7 +322,7 @@ static int _ep_port_ctl_op(ep_port_t* port_info,
     case EPOLL_CTL_DEL:
       return _ep_port_ctl_del(port_info, sock);
     default:
-      return_error(-1, ERROR_INVALID_PARAMETER);
+      return_set_error(-1, ERROR_INVALID_PARAMETER);
   }
 }
 
@@ -344,7 +344,7 @@ int ep_port_register_socket_handle(ep_port_t* port_info,
                                    SOCKET socket) {
   if (tree_add(
           &port_info->sock_tree, ep_sock_to_tree_node(sock_info), socket) < 0)
-    return_error(-1, ERROR_ALREADY_EXISTS);
+    return_set_error(-1, ERROR_ALREADY_EXISTS);
   return 0;
 }
 
@@ -356,7 +356,7 @@ void ep_port_unregister_socket_handle(ep_port_t* port_info,
 ep_sock_t* ep_port_find_socket(ep_port_t* port_info, SOCKET socket) {
   tree_node_t* tree_node = tree_find(&port_info->sock_tree, socket);
   if (tree_node == NULL)
-    return_error(NULL, ERROR_NOT_FOUND);
+    return_set_error(NULL, ERROR_NOT_FOUND);
   return ep_sock_from_tree_node(tree_node);
 }
 
diff --git a/src/reflock.c b/src/reflock.c
index 3c3db68..c4f10e7 100644
--- a/src/reflock.c
+++ b/src/reflock.c
@@ -22,7 +22,7 @@ int reflock_global_init(void) {
   NTSTATUS status =
       NtCreateKeyedEvent(&_keyed_event, ~(ACCESS_MASK) 0, NULL, 0);
   if (status != STATUS_SUCCESS)
-    return_error(-1, RtlNtStatusToDosError(status));
+    return_set_error(-1, RtlNtStatusToDosError(status));
   return 0;
 }
 
diff --git a/src/sock.c b/src/sock.c
index 0f1ab8b..3f6d871 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -38,7 +38,7 @@ typedef struct ep_sock {
 static inline ep_sock_t* _ep_sock_alloc(void) {
   ep_sock_t* sock_info = malloc(sizeof *sock_info);
   if (sock_info == NULL)
-    return_error(NULL, ERROR_NOT_ENOUGH_MEMORY);
+    return_set_error(NULL, ERROR_NOT_ENOUGH_MEMORY);
   return sock_info;
 }
 
@@ -54,7 +54,7 @@ static int _ep_sock_cancel_poll(ep_sock_t* sock_info) {
    * already completed. This is not a problem and we proceed normally. */
   if (!CancelIoEx(driver_handle, &sock_info->overlapped) &&
       GetLastError() != ERROR_NOT_FOUND)
-    return_error(-1);
+    return_map_error(-1);
 
   sock_info->poll_status = _POLL_CANCELLED;
   sock_info->pending_events = 0;
@@ -67,7 +67,7 @@ ep_sock_t* ep_sock_new(ep_port_t* port_info, SOCKET socket) {
   ep_sock_t* sock_info;
 
   if (socket == 0 || socket == INVALID_SOCKET)
-    return_error(NULL, ERROR_INVALID_HANDLE);
+    return_set_error(NULL, ERROR_INVALID_HANDLE);
 
   base_socket = ws_get_base_socket(socket);
   if (base_socket == INVALID_SOCKET)
@@ -243,7 +243,7 @@ int ep_sock_update(ep_port_t* port_info, ep_sock_t* sock_info) {
           return _ep_sock_delete(port_info, sock_info, false);
         default:
           /* Other errors are propagated to the caller. */
-          return_error(-1);
+          return_map_error(-1);
       }
     }
 
diff --git a/src/ws.c b/src/ws.c
index 857b7f0..73aa86a 100644
--- a/src/ws.c
+++ b/src/ws.c
@@ -18,7 +18,7 @@ int ws_global_init(void) {
 
   r = WSAStartup(MAKEWORD(2, 2), &wsa_data);
   if (r != 0)
-    return_error(-1, r);
+    return_set_error(-1, r);
 
   return 0;
 }
@@ -36,7 +36,7 @@ SOCKET ws_get_base_socket(SOCKET socket) {
                &bytes,
                NULL,
                NULL) == SOCKET_ERROR)
-    return_error(INVALID_SOCKET);
+    return_map_error(INVALID_SOCKET);
 
   return base_socket;
 }
@@ -51,7 +51,7 @@ ssize_t ws_get_protocol_catalog(WSAPROTOCOL_INFOW** infos_out) {
   for (;;) {
     infos = malloc(buffer_size);
     if (infos == NULL)
-      return_error(-1, ERROR_NOT_ENOUGH_MEMORY);
+      return_set_error(-1, ERROR_NOT_ENOUGH_MEMORY);
 
     count = WSAEnumProtocolsW(NULL, infos, &buffer_size);
     if (count == SOCKET_ERROR) {
@@ -59,7 +59,7 @@ ssize_t ws_get_protocol_catalog(WSAPROTOCOL_INFOW** infos_out) {
       if (WSAGetLastError() == WSAENOBUFS)
         continue; /* Try again with bigger buffer size. */
       else
-        return_error(-1);
+        return_map_error(-1);
     }
 
     *infos_out = infos;