diff --git a/httpie/uploads.py b/httpie/uploads.py
index 4a993b3a25..55e18de31b 100644
--- a/httpie/uploads.py
+++ b/httpie/uploads.py
@@ -188,6 +188,39 @@ def _prepare_file_for_upload(
         return file
 
 
+def _normalize_content_length_header(
+    value: Optional[Union[int, bytes, str]]
+) -> Optional[int]:
+    if value is None:
+        return None
+
+    if isinstance(value, int):
+        if value < 0:
+            raise ValueError('Invalid Content-Length header value')
+        return value
+
+    if isinstance(value, bytes):
+        try:
+            value = value.decode('ascii')
+        except UnicodeDecodeError as exc:
+            raise ValueError('Invalid Content-Length header value') from exc
+
+    if isinstance(value, str):
+        value = value.strip()
+        if value == '':
+            raise ValueError('Invalid Content-Length header value')
+
+    try:
+        normalized = int(value)
+    except (TypeError, ValueError) as exc:
+        raise ValueError('Invalid Content-Length header value') from exc
+
+    if normalized < 0:
+        raise ValueError('Invalid Content-Length header value')
+
+    return normalized
+
+
 def prepare_request_body(
     env: Environment,
     raw_body: Union[str, bytes, IO, 'MultipartEncoder', RequestDataDict],
@@ -210,6 +243,10 @@ def prepare_request_body(
         else:
             return body
 
+    content_length_header_value = _normalize_content_length_header(
+        content_length_header_value
+    )
+
     if is_file_like:
         return _prepare_file_for_upload(
             env,
diff --git a/tests/test_uploads.py b/tests/test_uploads.py
index e6bb80ac70..e16df2e3e9 100644
--- a/tests/test_uploads.py
+++ b/tests/test_uploads.py
@@ -12,6 +12,7 @@
 from httpie.client import FORM_CONTENT_TYPE
 from httpie.compat import is_windows
 from httpie.status import ExitStatus
+from httpie.uploads import prepare_request_body
 from .utils import (
     MockEnvironment, StdinBytesIO, http,
     HTTP_OK,
@@ -91,6 +92,34 @@ def test_chunked_raw(httpbin_with_chunked_support):
     assert 'Transfer-Encoding: chunked' in r
 
 
+def test_content_length_header_normalization(monkeypatch):
+    env = MockEnvironment(stdin_isatty=False)
+    captured = {}
+
+    def fake_prepare(env_arg, file, callback, chunked=False, content_length_header_value=None):
+        captured['value'] = content_length_header_value
+        return b''
+
+    monkeypatch.setattr('httpie.uploads._prepare_file_for_upload', fake_prepare)
+
+    prepare_request_body(
+        env,
+        StdinBytesIO(b'data'),
+        lambda chunk: chunk,
+        content_length_header_value=b'4',
+    )
+    assert captured['value'] == 4
+
+    with pytest.raises(ValueError):
+        prepare_request_body(
+            env,
+            StdinBytesIO(b'data'),
+            lambda chunk: chunk,
+            content_length_header_value=b'invalid',
+        )
+    assert captured['value'] == 4
+
+
 @contextlib.contextmanager
 def stdin_processes(httpbin, *args, warn_threshold=0.1):
     process_1 = subprocess.Popen(