diff --git a/httpx/_client.py b/httpx/_client.py
index bbeef7b5..17a71df8 100644
--- a/httpx/_client.py
+++ b/httpx/_client.py
@@ -1723,7 +1723,7 @@ class AsyncClient(BaseClient):
 
         if not isinstance(request.stream, AsyncByteStream):  # pragma: no cover
             raise RuntimeError(
-                "Attempted to send an sync request with an AsyncClient instance."
+                "Attempted to send a sync request with an AsyncClient instance."
             )
 
         with request_context(request=request):
diff --git a/httpx/_models.py b/httpx/_models.py
index 67d74bf8..2cc86321 100644
--- a/httpx/_models.py
+++ b/httpx/_models.py
@@ -964,7 +964,7 @@ class Response:
         Automatically called if the response body is read to completion.
         """
         if not isinstance(self.stream, SyncByteStream):
-            raise RuntimeError("Attempted to call an sync close on an async stream.")
+            raise RuntimeError("Attempted to call a sync close on an async stream.")
 
         if not self.is_closed:
             self.is_closed = True
@@ -1045,7 +1045,7 @@ class Response:
         if self.is_closed:
             raise StreamClosed()
         if not isinstance(self.stream, AsyncByteStream):
-            raise RuntimeError("Attempted to call an async iterator on an sync stream.")
+            raise RuntimeError("Attempted to call an async iterator on a sync stream.")
 
         self.is_stream_consumed = True
         self._num_bytes_downloaded = 0
@@ -1068,7 +1068,7 @@ class Response:
         Automatically called if the response body is read to completion.
         """
         if not isinstance(self.stream, AsyncByteStream):
-            raise RuntimeError("Attempted to call an async close on an sync stream.")
+            raise RuntimeError("Attempted to call an async close on a sync stream.")
 
         if not self.is_closed:
             self.is_closed = True