diff --git a/databricks/koalas/internal.py b/databricks/koalas/internal.py
index 16687e9e9e..16af2a94ec 100644
--- a/databricks/koalas/internal.py
+++ b/databricks/koalas/internal.py
@@ -534,13 +534,15 @@ def __init__(
 
         index_dtypes = [
             spark_type_to_pandas_dtype(spark_frame.select(scol).schema[0].dataType)
-            if dtype is None
+            if dtype is None or dtype == np.dtype("object")
             else dtype
             for dtype, scol in zip(index_dtypes, index_spark_columns)
         ]
 
         assert all(
-            as_spark_type(dtype, raise_error=False) is not None for dtype in index_dtypes
+            isinstance(dtype, Dtype.__args__)  # type: ignore
+            and (dtype == np.dtype("object") or as_spark_type(dtype, raise_error=False) is not None)
+            for dtype in index_dtypes
         ), index_dtypes
 
         self._index_dtypes = index_dtypes
@@ -593,13 +595,15 @@ def __init__(
 
         data_dtypes = [
             spark_type_to_pandas_dtype(spark_frame.select(scol).schema[0].dataType)
-            if dtype is None
+            if dtype is None or dtype == np.dtype("object")
             else dtype
             for dtype, scol in zip(data_dtypes, data_spark_columns)
         ]
 
         assert all(
-            as_spark_type(dtype, raise_error=False) is not None for dtype in data_dtypes
+            isinstance(dtype, Dtype.__args__)  # type: ignore
+            and (dtype == np.dtype("object") or as_spark_type(dtype, raise_error=False) is not None)
+            for dtype in data_dtypes
         ), data_dtypes
 
         self._data_dtypes = data_dtypes