diff --git a/requests/models.py b/requests/models.py
index 54428882..fe4bec1b 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -465,9 +465,11 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
 
     def prepare_content_length(self, body):
         if hasattr(body, 'seek') and hasattr(body, 'tell'):
+            curr_pos = body.tell()
             body.seek(0, 2)
-            self.headers['Content-Length'] = builtin_str(body.tell())
-            body.seek(0, 0)
+            end_pos = body.tell()
+            self.headers['Content-Length'] = builtin_str(max(0, end_pos - curr_pos))
+            body.seek(curr_pos, 0)
         elif body is not None:
             l = super_len(body)
             if l:
diff --git a/tests/test_requests.py b/tests/test_requests.py
index 282a6679..04a27a44 100755
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -482,6 +482,48 @@ class TestRequests:
         with pytest.raises(ValueError):
             requests.post(url, files=['bad file data'])
 
+    def test_POSTBIN_SEEKED_OBJECT_WITH_NO_ITER(self, httpbin):
+
+        class TestStream(object):
+            def __init__(self, data):
+                self.data = data.encode()
+                self.length = len(self.data)
+                self.index = 0
+
+            def __len__(self):
+                return self.length
+
+            def read(self, size=None):
+                if size:
+                    ret = self.data[self.index:self.index + size]
+                    self.index += size
+                else:
+                    ret = self.data[self.index:]
+                    self.index = self.length
+                return ret
+
+            def tell(self):
+                return self.index
+
+            def seek(self, offset, where=0):
+                if where == 0:
+                    self.index = offset
+                elif where == 1:
+                    self.index += offset
+                elif where == 2:
+                    self.index = self.length + offset
+
+        test = TestStream('test')
+        post1 = requests.post(httpbin('post'), data=test)
+        assert post1.status_code == 200
+        assert post1.json()['data'] == 'test'
+
+        test = TestStream('test')
+        test.seek(2)
+        post2 = requests.post(httpbin('post'), data=test)
+        assert post2.status_code == 200
+        assert post2.json()['data'] == 'st'
+
     def test_POSTBIN_GET_POST_FILES_WITH_DATA(self, httpbin):
 
         url = httpbin('post')