dmatrices raising “AssertionError”

Totally inexperienced user. My first Negative Binomial Regression. iPython on Google's Colab. I load the dataset as a pandas df. The features (and Target) in the formula below all appear in the df (which I named "dataset").

I also bring in

from patsy import dmatrices
import statsmodels.api as sm

however, when I

formula = """Target ~ MeanAge   + %White + %HHsNotWater + HHsIneq*10    + %NotSaLang + %male + %Informal + COGTACatG2B09 + %Poor + AGRating  """
data = dataset

response, predictors = dmatrices(formula, data, return_type='dataframe')
nb_results = sm.GLM(response, predictors, family=sm.families.NegativeBinomial(alpha=0.15)).fit()
print(nb_results.summary())

I simply get "AssertionError: ", and an arrow to line four (the one starting "response"). I have no idea how to remedy this, and cannot figure out if why this happens. Is it a Patsy issue? a Colab issue? A daft coding issue? Any sage guidance, please?

Start with the full traceback for the dmatrices(formula, data, return_type='dataframe') call and compare the formula terms with the dataset columns. Reproduce the assertion in the reported Colab or IPython setup using the shown formula and data, then document the minimal cause and a verified remedy.