diff --git a/CHANGES b/CHANGES
index b5933c1afa..3d64c12a2d 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,5 @@
+4519.	[port]		win32: handle ERROR_MORE_DATA. [RT #43534]
+
 4516.	[bug]		isc_socketmgr_renderjson was missing from the
 			windows build. [RT #43602]
 
diff --git a/lib/isc/win32/socket.c b/lib/isc/win32/socket.c
index 3e958bee4f..b358d98c8c 100644
--- a/lib/isc/win32/socket.c
+++ b/lib/isc/win32/socket.c
@@ -2489,14 +2489,16 @@ SocketIoThread(LPVOID ThreadContext) {
 
 		request = lpo->request_type;
 
-		errstatus = 0;
-		if (!bSuccess) {
+		if (!bSuccess)
+			errstatus = GetLastError();
+		else
+			errstatus = 0;
+		if (!bSuccess && errstatus != ERROR_MORE_DATA) {
 			isc_result_t isc_result;
 
 			/*
 			 * Did the I/O operation complete?
 			 */
-			errstatus = GetLastError();
 			isc_result = isc__errno2result(errstatus);
 
 			LOCK(&sock->lock);