diff --git a/src/header.zig b/src/header.zig
index 3963f89..8ab6785 100644
--- a/src/header.zig
+++ b/src/header.zig
@@ -1,10 +1,17 @@
 const std = @import("std");
 
-const HeaderSize = @bitSizeOf(Header) / 8;
+const HeaderSize = @sizeOf(Header);
 const Magic = [4]u8{ 0xf0, 0x9f, 0xa4, 0xb7 };
 const Version = 0;
 const Bom = 0x1234;
 
+const HeaderError = error{
+    InvalidMagic,
+    InvalidVersion,
+    InvalidBom,
+    TooManyShells,
+};
+
 const Header = packed struct {
     magic: [4]u8,
     version: u8,
@@ -21,10 +28,7 @@ const Header = packed struct {
     offset_groupmembers: u32,
     offset_additional_gids: u32,
 
-    // TODO(motiejus) should we be returning *Header instead? That way all
-    // stacks could be smaller by 48 bytes, and we would be referring to
-    // it's fields in the mmap'ed region.
-    pub fn init(blob: [HeaderSize]u8) Header {
+    pub fn init(blob: [HeaderSize]u8) HeaderError!Header {
         return @bitCast(Header, blob);
     }
 
@@ -58,6 +62,6 @@ test "header pack and unpack" {
     };
 
     const blob = header.asArray();
-    const header2 = Header.init(blob);
+    const header2 = try Header.init(blob);
     try testing.expectEqual(header, header2);
 }