Decompress NSData throws 5377 zlib

I try to decompress Data with zlib.
I cut a slice from a array of UInt8, which is compressed with zlib and try to decompress it.
Unfortunately it fails and I get a Error Domain=NSCocoaErrorDomain Code=5377 "(null)". I have no clue how to debug this.

I checked the header for compatibility and it is: 0x789c which seems right as statet from mwfearnley on stackoverflow.

        if isCompressed {
            let origDataArray: [UInt8] =
            let arraySlice: [UInt8] = Array(origDataArray[offset..<(offset + length)])
            let newData: Data = Data(arraySlice)
            let nsData: NSData = NSData(data: newData)

            // prints {length = 77, bytes = 0x789c6360 66fcc538 81819581 81a98b29 ... 00421e00 2d700cf5 }

            do {
                let dataDecompressed = try nsData.decompressed(using: .zlib)
            } catch {
                print("failed: ", error)

I’ve done a compression test like this:

 do {
            let dataTest = NSData(data: Data([0x8b, 0xaf, 0x19, 0x9b, 0xf1, 0xaa]))
            let compressed = try dataTest.compressed(using: .zlib)
            let decompressed = try compressed.decompressed(using: .zlib)
            print(compressed, decompressed)
        } catch {

And I got the following:

{length = 8, bytes = 0xeb5e2f39fbe32a00} {length = 6, bytes = 0x8baf199bf1aa}

In the compressed print (the first) there is no such header.
I think I miss something very fundamental..


Its working with SWCompression Library though.. weird


Got it working.
Had to cut the first two header bytes away.

// Instead of
let arraySlice: [UInt8] = Array(origDataArray[offset..<(offset + length)])

// do 
let arraySlice: [UInt8] = Array(origDataArray[offset+2..<(offset + length)])