diff --git a/httpx/_transports/default.py b/httpx/_transports/default.py
index f2f9abc9db..3e4ed78394 100644
--- a/httpx/_transports/default.py
+++ b/httpx/_transports/default.py
@@ -132,7 +132,7 @@ def __init__(
     ) -> None:
         ssl_context = create_ssl_context(verify=verify, cert=cert, trust_env=trust_env)
 
-        if proxy and isinstance(proxy, str):
+        if proxy is not None and isinstance(proxy, str):
             proxy = Proxy(url=proxy)
 
         if proxy is None:
@@ -274,7 +274,7 @@ def __init__(
     ) -> None:
         ssl_context = create_ssl_context(verify=verify, cert=cert, trust_env=trust_env)
 
-        if proxy and isinstance(proxy, str):
+        if proxy is not None and isinstance(proxy, str):
             proxy = Proxy(url=proxy)
 
         if proxy is None: