diff --git a/interface/fs.xpc b/interface/fs.xpc
index 16d273b..1d42ccd 100644
--- a/interface/fs.xpc
+++ b/interface/fs.xpc
@@ -6,4 +6,4 @@ func close[1]() -> (err: int);
 func read[2](count: size) -> (err: int, nr_read: size, data: buffer);
 func write[3](data: buffer) -> (err: int, nr_written: size);
 
-func map[4](prot: int, flags: int) -> (vmo: handle);
+func map[4](prot: int, flags: int) -> (err: int, vmo: handle);