diff --git a/std/os/index.zig b/std/os/index.zig
index c9103ff93b5601ac39bf640609284e023c5cde1a..6f5631c1202d07bcb21cf5947006d70e203ed8a4 100644
--- a/std/os/index.zig
+++ b/std/os/index.zig
@@ -783,7 +783,7 @@ pub fn makeDirWindows(allocator: &Allocator, dir_path: []const u8) -> %void {
 }
 
 pub fn makeDirPosix(allocator: &Allocator, dir_path: []const u8) -> %void {
-    const path_buf = cstr.addNullByte(allocator, dir_path);
+    const path_buf = %return cstr.addNullByte(allocator, dir_path);
     defer allocator.free(path_buf);
 
     const err = posix.getErrno(posix.mkdir(path_buf.ptr, 0o755));