diff --git a/httpx/_multipart.py b/httpx/_multipart.py
index 1d46d96a98..446f4ad2df 100644
--- a/httpx/_multipart.py
+++ b/httpx/_multipart.py
@@ -122,14 +122,14 @@ def __init__(self, name: str, value: FileTypes) -> None:
             # requests does the opposite (it overwrites the header with the 3rd tuple element)
             headers["Content-Type"] = content_type
 
-        if "b" not in getattr(fileobj, "mode", "b"):
-            raise TypeError(
-                "Multipart file uploads must be opened in binary mode, not text mode."
-            )
         if isinstance(fileobj, io.StringIO):
             raise TypeError(
                 "Multipart file uploads require 'io.BytesIO', not 'io.StringIO'."
             )
+        if isinstance(fileobj, io.TextIOBase):
+            raise TypeError(
+                "Multipart file uploads must be opened in binary mode, not text mode."
+            )
 
         self.filename = filename
         self.file = fileobj