diff --git a/std/os/darwin.zig b/std/os/darwin.zig
index c64ce807b6..3e883abbab 100644
--- a/std/os/darwin.zig
+++ b/std/os/darwin.zig
@@ -665,7 +665,7 @@ pub fn pwrite(fd: i32, buf: [*]const u8, nbyte: usize, offset: u64) usize {
 
 pub fn mmap(address: ?[*]u8, length: usize, prot: usize, flags: u32, fd: i32, offset: isize) usize {
     const ptr_result = c.mmap(
-        @ptrCast(*c_void, address),
+        @ptrCast(?*c_void, address),
         length,
         @bitCast(c_int, @intCast(c_uint, prot)),
         @bitCast(c_int, c_uint(flags)),