diff --git a/docs/source/internal/utils.rst b/docs/source/internal/utils.rst
index 31e0d39..2b2638e 100644
--- a/docs/source/internal/utils.rst
+++ b/docs/source/internal/utils.rst
@@ -25,13 +25,8 @@
 
     "E121,W123,F904"
     "E121,\nW123,\nF804"
-    "E121,\n\tW123,\n\tF804"
-
-Or it will take a list of strings (potentially with whitespace) such as
-
-.. code-block:: python
-
-    ["   E121\n", "\t\nW123   ", "\n\tF904\n    "]
+    " E121,\n\tW123,\n\tF804 "
+    " E121\n\tW123 \n\tF804"
 
 And converts it to a list that looks as follows
 
diff --git a/src/flake8/utils.py b/src/flake8/utils.py
index cf88648..06a1db1 100644
--- a/src/flake8/utils.py
+++ b/src/flake8/utils.py
@@ -24,11 +24,11 @@ string_types = (str, type(u""))
 
 
 def parse_comma_separated_list(value, regexp=COMMA_SEPARATED_LIST_RE):
-    # type: (Union[Sequence[str], str], Pattern[str]) -> List[str]
+    # type: (Optional[str], Pattern[str]) -> List[str]
     """Parse a comma-separated list.
 
     :param value:
-        String or list of strings to be parsed and normalized.
+        String to be parsed and normalized.
     :param regexp:
         Compiled regular expression used to split the value when it is a
         string.
@@ -46,10 +46,8 @@ def parse_comma_separated_list(value, regexp=COMMA_SEPARATED_LIST_RE):
     if not value:
         return []
 
-    if isinstance(value, string_types):
-        value = regexp.split(value)
-
-    item_gen = (item.strip() for item in value)
+    separated = regexp.split(value)
+    item_gen = (item.strip() for item in separated)
     return [item for item in item_gen if item]
 
 
diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
index 20c5fb9..b4bd1a1 100644
--- a/tests/unit/test_utils.py
+++ b/tests/unit/test_utils.py
@@ -22,11 +22,6 @@ RELATIVE_PATHS = ["flake8", "pep8", "pyflakes", "mccabe"]
     ("E123,W234,,E206,,", ["E123", "W234", "E206"]),
     ("E123, W234,, E206,,", ["E123", "W234", "E206"]),
     ("E123,,W234,,E206,,", ["E123", "W234", "E206"]),
-    (["E123", "W234", "E206"], ["E123", "W234", "E206"]),
-    (["E123", "\n\tW234", "\n    E206"], ["E123", "W234", "E206"]),
-    (["E123", "\n\tW234", "\n    E206", "\n"], ["E123", "W234", "E206"]),
-    (["E123", "\n\tW234", "", "\n    E206", "\n"], ["E123", "W234", "E206"]),
-    (["E123", "\n\tW234", "", "\n    E206", ""], ["E123", "W234", "E206"]),
 ])
 def test_parse_comma_separated_list(value, expected):
     """Verify that similar inputs produce identical outputs."""