title: Error: "base_score > 0.0f && base_score < 1.0f base_score must be in (0,1) for logistic loss" on xgboost 0.7
body: <p>I have a binary classification problem, and I wanted to try : XGBoost model since I have good results with GradientBoosting (sklearn) model on the same data set.</p>
  
  <pre><code>import xgboost as xgb
  
  XGB = xgb.XGBClassifier()
  model = XGB.fit(X_train, y_train)
  </code></pre>
  
  <p>But I have an error that I don't understand </p>
  
  <pre><code>XGBoostError: b'[11:52:35] src/objective/regression_obj.cc:48: Check failed: base_score &gt; 0.0f &amp;&amp; base_score &lt; 1.0f base_score must be in (0,1) for logistic loss
  
  Stack trace returned 10 entries:
  [bt] (0) /home/ilb/anaconda3/lib/python3.6/site-packages/xgboost/./lib/libxgboost.so(_ZN4dmlc15LogMessageFatalD1Ev+0x29) [0x7f3bd4d15299]
  [bt] (1) /home/ilb/anaconda3/lib/python3.6/site-packages/xgboost/./lib/libxgboost.so(_ZN7xgboost3obj18LogisticRegression12ProbToMarginEf+0x7e) [0x7f3bd4d9116e]
  [bt] (2) /home/ilb/anaconda3/lib/python3.6/site-packages/xgboost/./lib/libxgboost.so(_ZN7xgboost11LearnerImpl13LazyInitModelEv+0x264) [0x7f3bd4d204a4]
  [bt] (3) /home/ilb/anaconda3/lib/python3.6/site-packages/xgboost/./lib/libxgboost.so(XGBoosterUpdateOneIter+0x4a) [0x7f3bd4e69afa]
  [bt] (4) /home/ilb/anaconda3/lib/python3.6/lib-dynload/_ctypes.cpython-36m-x86_64-linux-gnu.so(ffi_call_unix64+0x4c) [0x7f3ee8db6550]
  [bt] (5) /home/ilb/anaconda3/lib/python3.6/lib-dynload/_ctypes.cpython-36m-x86_64-linux-gnu.so(ffi_call+0x1f5) [0x7f3ee8db5cf5]
  [bt] (6) /home/ilb/anaconda3/lib/python3.6/lib-dynload/_ctypes.cpython-36m-x86_64-linux-gnu.so(_ctypes_callproc+0x3dc) [0x7f3ee8dad83c]
  [bt] (7) /home/ilb/anaconda3/lib/python3.6/lib-dynload/_ctypes.cpython-36m-x86_64-linux-gnu.so(+0x9da3) [0x7f3ee8da5da3]
  [bt] (8) /home/ilb/anaconda3/bin/../lib/libpython3.6m.so.1.0(_PyObject_FastCallDict+0x9e) [0x7f3f1f69792e]
  [bt] (9) /home/ilb/anaconda3/bin/../lib/libpython3.6m.so.1.0(+0x147d1b) [0x7f3f1f773d1b]
  </code></pre>