Explicit integer type cast

Author: clbarnes

pandas/core/arrays/categorical.py

@@ -448,9 +448,8 @@ def from_dummies(
         #  010            020    2    1
         #  001 * 1,2,3 => 003 -> 3 -> 2 = correct codes
         #  100            100    1    0
-        codes = (df * mult_by).sum(axis=1) - 1
-        codes[codes.isna()] = -1
-        return cls.from_codes(codes, df.columns.values, ordered=ordered)
+        codes = ((df * mult_by).sum(axis=1) - 1).astype("Int64")
+        return cls.from_codes(codes.fillna(-1), df.columns.values, ordered=ordered)
 
     def to_dummies(self, na_column=None) -> "DataFrame":
         """