Finding cause of newRenderPipelineStateWithDescriptor failure - ios

Question

I'm trying to create a pipeline state in swift and the app crashes with SIGABRT. I've put the call to newRenderPipelineStateWithDescriptor within a try catch block. Why is it not caching it?
Here's the code,
let defaultLibrary = device.newDefaultLibrary()!
let fragmentProgram = defaultLibrary.newFunctionWithName("passThroughFragment")!
let vertexProgram = defaultLibrary.newFunctionWithName("passGeometry")!
// check TexturedVertex
let vertexDesc = MTLVertexDescriptor()
vertexDesc.attributes[0].format = .Float3
vertexDesc.attributes[0].offset = 0
vertexDesc.attributes[0].bufferIndex = 0
vertexDesc.attributes[1].format = .Float3
vertexDesc.attributes[0].offset = sizeof(Vec3)
vertexDesc.attributes[0].bufferIndex = 0
vertexDesc.attributes[2].format = .Float2
vertexDesc.attributes[0].offset = sizeof(Vec3) * 2
vertexDesc.attributes[0].bufferIndex = 0
vertexDesc.layouts[0].stepFunction = .PerVertex
vertexDesc.layouts[0].stride = sizeof(TexturedVertex)
let pipelineStateDescriptor = MTLRenderPipelineDescriptor()
pipelineStateDescriptor.vertexFunction = vertexProgram
pipelineStateDescriptor.fragmentFunction = fragmentProgram
//pipelineStateDescriptor.vertexDescriptor = vertexDesc
pipelineStateDescriptor.colorAttachments[0].pixelFormat = view.colorPixelFormat
pipelineStateDescriptor.colorAttachments[0].blendingEnabled = false
pipelineStateDescriptor.sampleCount = view.sampleCount
do {
// SIGABRT will happen here when enabling .vertexDescriptor = vertexDesc
try pipelineState = device.newRenderPipelineStateWithDescriptor(pipelineStateDescriptor)
} catch let error {
print("Failed to create pipeline state, error \(error)")
}
The code above doesn't crash, until I enable this line,
pipelineStateDescriptor.vertexDescriptor = vertexDesc
To give a bit of background, my vertex shader used to receive a packed_float4 buffer as input but I'm trying to update it to a struct, as defined in the MTLVertexDescriptor above.
The frustrating thing is that the app just crashes without giving any hints of what's wrong.
Is there any way of get error messages when calling device.newRenderPipelineStateWithDescriptor?
Edit:
This fixes the crash,
vertexDesc.attributes[0].format = .Float3
vertexDesc.attributes[0].offset = 0
vertexDesc.attributes[0].bufferIndex = 0
vertexDesc.attributes[1].format = .Float3
vertexDesc.attributes[1].offset = sizeof(Vec3)
vertexDesc.attributes[1].bufferIndex = 0
vertexDesc.attributes[2].format = .Float2
vertexDesc.attributes[2].offset = sizeof(Vec3) * 2
vertexDesc.attributes[2].bufferIndex = 0
But apparently there's no way to make newRenderPipelineStateWithDescriptor to throw an exception because of that at the moment.