diff --git a/requests/utils.py b/requests/utils.py
index e37b9109..dfeb77d9 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -367,20 +367,13 @@ def get_encoding_from_headers(headers):
 
 def stream_decode_response_unicode(iterator, r):
     """Stream decodes a iterator."""
-    encoding = r.encoding
 
-    if encoding is None:
-        encoding = r.apparent_encoding
-
-    try:
-        decoder = codecs.getincrementaldecoder(encoding)(errors='replace')
-    except (LookupError, TypeError):
-        # A LookupError is raised if the encoding was not found which could
-        # indicate a misspelling or similar mistake.
-        #
-        # A TypeError can be raised if encoding is None
-        raise UnicodeError("Unable to decode contents with encoding %s." % encoding)
+    if r.encoding is None:
+        for item in iterator:
+            yield item
+        return
 
+    decoder = codecs.getincrementaldecoder(r.encoding)(errors='replace')
     for chunk in iterator:
         rv = decoder.decode(chunk)
         if rv:
diff --git a/tests/test_requests.py b/tests/test_requests.py
index 3d959a4b..efbc7ed0 100755
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -973,13 +973,6 @@ class TestRequests:
         chunks = r.iter_content(decode_unicode=True)
         assert all(isinstance(chunk, str) for chunk in chunks)
 
-        # check for encoding value of None
-        r = requests.Response()
-        r.raw = io.BytesIO(b'the content')
-        r.encoding = None
-        chunks = r.iter_content(decode_unicode=True)
-        assert all(isinstance(chunk, str) for chunk in chunks)
-
     def test_response_reason_unicode(self):
         # check for unicode HTTP status
         r = requests.Response()