diff --git a/requests/utils.py b/requests/utils.py
index 769b4012..dea323ef 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -384,6 +384,8 @@ def stream_decode_response_unicode(iterator, r):
 def iter_slices(string, slice_length):
     """Iterate over slices of a string."""
     pos = 0
+    if slice_length is None or slice_length <= 0: 
+        slice_length = len(string)
     while pos < len(string):
         yield string[pos:pos + slice_length]
         pos += slice_length
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 17149d26..ab5c2e37 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -378,10 +378,16 @@ def test_get_encoding_from_headers(value, expected):
         ('', 0),
         ('T', 1),
         ('Test', 4),
+        ('Cont', 0),
+        ('Other', -5),
+        ('Content', None),
     ))
 def test_iter_slices(value, length):
-    assert len(list(iter_slices(value, 1))) == length
-
+    if length is None or (length <= 0 and len(value) > 0):
+        # Reads all content at once
+        assert len(list(iter_slices(value, length))) == 1
+    else:
+        assert len(list(iter_slices(value, 1))) == length
 
 @pytest.mark.parametrize(
     'value, expected', (