How to properly use skinningJointMatrices, in SceneKit+Custom metal shader ?

I'm animating body+ face moments using skeleton animation , SceneKit and Metal custom shader.

I need to deform some vertices in the vertex shader, therefore I need skinningJointMatrices in the vertex shader.

However if I just add  the line  

float4 skinningJointMatrices[183];

in the NodeBuffer :

struct NodeBuffer {
  float4x4 inverseModelTransform;
  float4x4 inverseModelViewTransform;

  float4x4 modelTransform;
  float4x4 modelViewProjectionTransform;
  float4x4 modelViewTransform;
  float4x4 normalTransform;
  float2x3 boundingBox;
  float4 skinningJointMatrices[765]; 

};

I get the following assertion:

[SceneKit] Assertion 'C3DSkinnerGetEffectiveCalculationMode(skinner, C3DNodeGetGeometry(context->_nodeUniforms.instanceNode)) == kC3DSkinnerCalculationModeGPUVertexFunction' failed. skinningJointMatrices should only be used when skinning is done in the vertex function

Is there a way to workaround this assert?

The code seems to be working fine although the assertion.