AVAudioEngine (OSX) crashes when trying to use audio input

On OSX 10.13.5 I am trying to get the AVAudioEngine working I am experiencing crashes...

I am trying to get the engine running using an external thunderbolt sound card (UAD Apollo).

I am trying to only build a simple graph to get things working:

audioInput -> NI-Reaktor -> audioOutput

The AVAudioEngine prepares and runs correctly when I do not connect the inputNode. HOWEVER, the processing plugin (Reaktor) receives garbarge buffers at it's inputs... (LOUD!)

Does anyone have some experience they could share with me please?

Thanks in advance,

|K<

the exception raised:

*** Terminating app due to uncaught exception 'com.apple.coreaudio.avfaudio', reason: 'required condition is false: 0 == inCurrNode.NumberOfValidConnections(kUpstream)'

The engine connection code I've got looks like this:

    AVAudioFormat* outputFormat = [self.engine.outputNode outputFormatForBus:0];
    AVAudioFormat* inputsFormat = [self.engine.inputNode inputFormatForBus:0];
    AVAudioNode* audioOutputNode = self.engine.outputNode;
    AVAudioNode* audioInputNode = self.engine.inputNode;
    AVAudioNode* reaktorNode = self.reaktorAU;

    [self.engine connect:audioInputNode to:reaktorNode fromBus:0 toBus:0 format:nil];// crashes
//    [self.engine connect:audioInputNode to:reaktorNode format:inputsFormat];          crashes
//    [self.engine connect:audioInputNode to:reaktorNode format:nil];                   crashes
//    [self.engine connect:audioInputNode to:reaktorNode format:outputFormat];          crashes
    [self.engine connect:reaktorNode to:audioOutputNode format:outputFormat];

    [self.engine prepare]; // this is where the crash happens.

the resulting AUGraph looks like this:

(lldb) po self.engine

________ GraphDescription ________
AVAudioEngineGraph 0x1006135d0: initialized = 0, running = 0, number of nodes = 3

  ******** output chain ********

  node 0x6000000e1c80 {'auou' 'ahal' 'appl'}, 'U'
  inputs = 1
  (bus0, en1) <- (bus0) 0x6000000e3e00, {'aumu' 'NiR6' '-NI-'}, [10 ch,  96000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]

  node 0x6000000e3e00 {'aumu' 'NiR6' '-NI-'}, 'U'
  inputs = 1
  (bus0, en1) <- (bus1) 0x6080000e3c80, {'auou' 'ahal' 'appl'}, [ 2 ch,  96000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]
  outputs = 1
  (bus0, en1) -> (bus0) 0x6000000e1c80, {'auou' 'ahal' 'appl'}, [10 ch,  96000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]

  node 0x6080000e3c80 {'auou' 'ahal' 'appl'}, 'U'
  outputs = 2
  (bus0, en0) -> (bus0) 0x0, {}, [ 2 ch,  96000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]
  (bus1, en1) -> (bus0) 0x6000000e3e00, {'aumu' 'NiR6' '-NI-'}, [ 2 ch,  96000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]

  ******** input chain ********

  node 0x6080000e3c80 {'auou' 'ahal' 'appl'}, 'U'
  outputs = 2
  (bus0, en0) -> (bus0) 0x0, {}, [ 2 ch,  96000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]
  (bus1, en1) -> (bus0) 0x6000000e3e00, {'aumu' 'NiR6' '-NI-'}, [ 2 ch,  96000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]

  node 0x6000000e3e00 {'aumu' 'NiR6' '-NI-'}, 'U'
  inputs = 1
  (bus0, en1) <- (bus1) 0x6080000e3c80, {'auou' 'ahal' 'appl'}, [ 2 ch,  96000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]
  outputs = 1
  (bus0, en1) -> (bus0) 0x6000000e1c80, {'auou' 'ahal' 'appl'}, [10 ch,  96000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]

  node 0x6000000e1c80 {'auou' 'ahal' 'appl'}, 'U'
  inputs = 1
  (bus0, en1) <- (bus0) 0x6000000e3e00, {'aumu' 'NiR6' '-NI-'}, [10 ch,  96000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]
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