AsynchronousXopt.to_json() broken

[ChristopherMayes]

#from xopt import Xopt # Works
from xopt import AsynchronousXopt as Xopt # Error

YAML = """
generator:
  name: random

evaluator:
  function: xopt.resources.test_functions.tnk.evaluate_TNK

vocs:
  variables:
    x1: [0, 3.14159]
    x2: [0, 3.14159]
  objectives: {y1: MINIMIZE}

"""
X = Xopt(YAML)
X.step()
X.step()
X.to_json()

...

ValueError: DataFrame index must be unique for orient='columns'.