diff --git a/os/io.c b/os/io.c
index cf5e8f933..8181a8639 100644
--- a/os/io.c
+++ b/os/io.c
@@ -102,12 +102,17 @@ typedef struct _connectionOutput {
 static ConnectionInputPtr AllocateInputBuffer(void);
 static ConnectionOutputPtr AllocateOutputBuffer(void);
 
-/* check for both EAGAIN and EWOULDBLOCK, because some supposedly POSIX
+/* If EAGAIN and EWOULDBLOCK are distinct errno values, then we check errno
+ * for both EAGAIN and EWOULDBLOCK, because some supposedly POSIX
  * systems are broken and return EWOULDBLOCK when they should return EAGAIN
  */
 #ifndef WIN32
-#define ETEST(err) (err == EAGAIN || err == EWOULDBLOCK)
-#else                           /* WIN32 The socket errorcodes differ from the normal errors */
+# if (EAGAIN != EWOULDBLOCK)
+#  define ETEST(err) (err == EAGAIN || err == EWOULDBLOCK)
+# else
+#  define ETEST(err) (err == EAGAIN)
+# endif
+#else   /* WIN32 The socket errorcodes differ from the normal errors */
 #define ETEST(err) (err == EAGAIN || err == WSAEWOULDBLOCK)
 #endif