PatsyError: Фактор ошибки при оценке: NameError:

# For reading data set
# importing necessary libraries
import pandas as pd 
import numpy as np
import matplotlib.pyplot as plt

# reading a csv file using pandas library
calo=pd.read_csv("/Users/Sanjeev/Desktop/Excel R Assignments/Simple Linear Regression/calories_consumed.csv")
calo.columns = ['wt','cal']

np.corrcoef(calo.wt,calo.cal)

plt.plot(calo.wt,calo.cal,"bo");plt.xlabel("WEIGHT");plt.ylabel("CALORIES")

# For preparing linear regression model we need to import the statsmodels.formula.api
import statsmodels.formula.api as smf
model = smf.ols("wt~cal",data=calo).fit()

# For getting coefficients of the varibles used in equation
model.params

# P-values for the variables and R-squared value for prepared model
model.summary()

model.conf_int(0.05) # 95% confidence interval

pred = model.predict(pd.DataFrame(calo['wt']))

Traceback (most recent call last):

  File "<ipython-input-43-4fcbf1ee1921>", line 1, in <module>
    pred = model.predict(pd.DataFrame(calo['wt']))

  File "/anaconda3/lib/python3.7/site-packages/statsmodels/base/model.py", line 837, in predict
    exog = dmatrix(design_info, exog, return_type = "dataframe")

  File "/anaconda3/lib/python3.7/site-packages/patsy/highlevel.py", line 291, in dmatrix
    NA_action, return_type)

  File "/anaconda3/lib/python3.7/site-packages/patsy/highlevel.py", line 169, in _do_highlevel_design
    return_type=return_type)

  File "/anaconda3/lib/python3.7/site-packages/patsy/build.py", line 888, in build_design_matrices
    value, is_NA = _eval_factor(factor_info, data, NA_action)

  File "/anaconda3/lib/python3.7/site-packages/patsy/build.py", line 63, in _eval_factor
    result = factor.eval(factor_info.state, data)

  File "/anaconda3/lib/python3.7/site-packages/patsy/eval.py", line 566, in eval
    data)

  File "/anaconda3/lib/python3.7/site-packages/patsy/eval.py", line 551, in _eval
    inner_namespace=inner_namespace)

  File "/anaconda3/lib/python3.7/site-packages/patsy/compat.py", line 43, in call_and_wrap_exc
    exec("raise new_exc from e")

  File "<string>", line 1, in <module>

PatsyError: Error evaluating factor: NameError: name 'cal' is not defined
    wt~cal
       ^^^