onnx: pytest error "No Schema registered for Abs with domain_version of 6"

After successful installation from source on Windows (with pip install -e onnx), I’m trying to run the tests but getting “No Schema registered for Abs with domain_version of 6” error. Am I missing something?

(test-py36) c:\repos\onnx>pytest -x
============================= test session starts =============================
platform win32 -- Python 3.6.5, pytest-3.5.1, py-1.5.3, pluggy-0.6.0
rootdir: c:\repos\onnx, inifile: setup.cfg
plugins: cov-2.5.1, nbval-0.9.0
collected 1520 items

onnx\test\backend_shape_inference_test.py F

================================== FAILURES ===================================
____________________ OnnxBackendNodeModelTest.test_abs_cpu ____________________

test_self = <backend_shape_inference_test.OnnxBackendNodeModelTest testMethod=test_abs_cpu>
device = 'CPU'

    def run(test_self, device):
        if model_test.model_dir is None:
            model_dir = self._prepare_model_data(model_test)
        else:
            model_dir = model_test.model_dir
        model_pb_path = os.path.join(model_dir, 'model.onnx')
        model = onnx.load(model_pb_path)
        model_marker[0] = model
>       prepared_model = self.backend.prepare(model, device)

onnx\backend\test\runner\__init__.py:237:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
onnx\test\backend_shape_inference_test.py:22: in prepare
    super(DummyBackend, cls).prepare(model, device, **kwargs)
onnx\backend\base.py:55: in prepare
    onnx.checker.check_model(model)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

model = ir_version: 3
producer_name: "backend-test"
graph {
  node {
    input: "x"
    output: "y"
    op_type: "Abs"
  }
  n...    }
          dim {
            dim_value: 5
          }
        }
      }
    }
  }
}
opset_import {
  version: 6
}


    def check_model(model):
>       C.check_model(model.SerializeToString())
E       onnx.onnx_cpp2py_export.checker.ValidationError: No Schema registered for Abs with domain_version of 6
E
E       ==> Context: Bad node spec: input: "x" output: "y" op_type: "Abs"

onnx\checker.py:76: ValidationError