diff --git a/src/base64io/__init__.py b/src/base64io/__init__.py
index a952212..d399080 100644
--- a/src/base64io/__init__.py
+++ b/src/base64io/__init__.py
@@ -23,7 +23,7 @@
 
 try:  # Python 3.5.0 and 3.5.1 have incompatible typing modules
     from types import TracebackType  # noqa pylint: disable=unused-import
-    from typing import IO, Iterable, List, Type, Optional  # noqa pylint: disable=unused-import
+    from typing import Union, IO, Iterable, List, Type, Optional, AnyStr  # noqa pylint: disable=unused-import
 except ImportError:  # pragma: no cover
     # We only actually need these imports when running the mypy checks
     pass
@@ -209,7 +209,7 @@ def writelines(self, lines):
             self.write(line)
 
     def _read_additional_data_removing_whitespace(self, data, total_bytes_to_read):
-        # type: (bytes, int) -> bytes
+        # type: (AnyStr, int) -> AnyStr
         """Read additional data from wrapped stream until we reach the desired number of bytes.
 
         .. note::
@@ -226,9 +226,10 @@ def _read_additional_data_removing_whitespace(self, data, total_bytes_to_read):
             # case the base64 module happily removes any whitespace.
             return data
 
-        _data_buffer = io.BytesIO()
-        _data_buffer.write(b"".join(data.split()))
-        _remaining_bytes_to_read = total_bytes_to_read - _data_buffer.tell()
+        _data_buffer = io.BytesIO() if isinstance(data, bytes) else io.StringIO()
+        join_char = b'' if isinstance(data, bytes) else u''
+        _data_buffer.write(join_char.join(data.split()))  # type: ignore
+        _remaining_bytes_to_read = total_bytes_to_read - _data_buffer.tell()  # type: ignore
 
         while _remaining_bytes_to_read > 0:
             _raw_additional_data = self.__wrapped.read(_remaining_bytes_to_read)
@@ -236,9 +237,9 @@ def _read_additional_data_removing_whitespace(self, data, total_bytes_to_read):
                 # No more data to read from wrapped stream.
                 break
 
-            _data_buffer.write(b"".join(_raw_additional_data.split()))
-            _remaining_bytes_to_read = total_bytes_to_read - _data_buffer.tell()
-        return _data_buffer.getvalue()
+            _data_buffer.write(join_char.join(_raw_additional_data.split()))  # type: ignore
+            _remaining_bytes_to_read = total_bytes_to_read - _data_buffer.tell()  # type: ignore
+        return _data_buffer.getvalue()  # type: ignore
 
     def read(self, b=-1):
         # type: (int) -> bytes
@@ -273,7 +274,10 @@ def read(self, b=-1):
         data = self.__wrapped.read(_bytes_to_read)
         # Remove whitespace from read data and attempt to read more data to get the desired
         # number of bytes.
-        if any([char.encode("utf-8") in data for char in string.whitespace]):
+        whitespace = string.whitespace.encode("utf-8") if isinstance(data, bytes) \
+            else string.whitespace  # type: Union[bytes, str]
+
+        if any([char in data for char in whitespace]):
             data = self._read_additional_data_removing_whitespace(data, _bytes_to_read)
 
         results = io.BytesIO()
diff --git a/test/unit/test_base64_stream.py b/test/unit/test_base64_stream.py
index de8410c..4492516 100644
--- a/test/unit/test_base64_stream.py
+++ b/test/unit/test_base64_stream.py
@@ -154,6 +154,22 @@ def test_base64io_decode(bytes_to_generate, bytes_per_round, number_of_rounds, t
     assert test == plaintext_source[:total_bytes_to_expect]
 
 
+@pytest.mark.parametrize(
+    "bytes_to_generate, bytes_per_round, number_of_rounds, total_bytes_to_expect", build_test_cases()
+)
+def test_base64io_decode_str(bytes_to_generate, bytes_per_round, number_of_rounds, total_bytes_to_expect):
+    plaintext_source = os.urandom(bytes_to_generate)
+    plaintext_b64 = io.StringIO(base64.b64encode(plaintext_source).decode('ascii'))
+    plaintext_wrapped = Base64IO(plaintext_b64)
+
+    test = b""
+    for _round in range(number_of_rounds):
+        test += plaintext_wrapped.read(bytes_per_round)
+
+    assert len(test) == total_bytes_to_expect
+    assert test == plaintext_source[:total_bytes_to_expect]
+
+
 @pytest.mark.parametrize(
     "bytes_to_generate, bytes_per_round, number_of_rounds, total_bytes_to_expect", build_test_cases()
 )
@@ -297,6 +313,14 @@ def test_base64io_decode_with_whitespace(plaintext_source, b64_plaintext_with_wh
     assert test == plaintext_source[:read_bytes]
 
 
+@pytest.mark.parametrize("plaintext_source, b64_plaintext_with_whitespace, read_bytes", build_whitespace_testcases())
+def test_base64io_decode_with_whitespace_str(plaintext_source, b64_plaintext_with_whitespace, read_bytes):
+    with Base64IO(io.StringIO(b64_plaintext_with_whitespace.decode('ascii'))) as decoder:
+        test = decoder.read(read_bytes)
+
+    assert test == plaintext_source[:read_bytes]
+
+
 @pytest.mark.parametrize(
     "plaintext_source, b64_plaintext_with_whitespace, read_bytes", ((b"\x00\x00\x00", b"AAAA", 3),)
 )