BUG/REG: fix float64index -> mixed float assignment

doc/source/v0.14.1.txt

@@ -120,3 +120,6 @@ Bug Fixes
 - Bug all ``StringMethods`` now work on empty Series (:issue:`7242`)
 - Fix delegation of `read_sql` to `read_sql_query` when query does not contain
  'select' (:issue:`7324`).
+- Bug where a string column name assignment to a ``DataFrame`` with a
+  ``Float64Index`` raised a ``TypeError`` during a call to ``np.isnan``
+  (:issue:`7366`).

pandas/core/index.py

@@ -2060,11 +2060,14 @@ def __contains__(self, other):
             return False
 
     def get_loc(self, key):
-        if np.isnan(key):
-            try:
-                return self._nan_idxs.item()
-            except ValueError:
-                return self._nan_idxs
+        try:
+            if np.isnan(key):
+                try:
+                    return self._nan_idxs.item()
+                except ValueError:
+                    return self._nan_idxs
+        except (TypeError, NotImplementedError):
+            pass
         return super(Float64Index, self).get_loc(key)
 
     @property

pandas/tests/test_indexing.py

@@ -3561,6 +3561,16 @@ def f():
 
         warnings.filterwarnings(action='ignore', category=FutureWarning)
 
+    def test_float_index_to_mixed(self):
+        df = DataFrame({0.0: np.random.rand(10),
+                        1.0: np.random.rand(10)})
+        df['a'] = 10
+        tm.assert_frame_equal(DataFrame({0.0: df[0.0],
+                                         1.0: df[1.0],
+                                         'a': [10] * 10}),
+                              df)
+
+
 if __name__ == '__main__':
     import nose
     nose.runmodule(argv=[__file__, '-vvs', '-x', '--pdb', '--pdb-failure'],