diff --git a/pymysql/err.py b/pymysql/err.py
index e93ba9be..8ca23655 100644
--- a/pymysql/err.py
+++ b/pymysql/err.py
@@ -100,5 +100,7 @@ def _map_error(exc, *errors):
 def raise_mysql_exception(data):
     errno = struct.unpack('<h', data[1:3])[0]
     errval = data[9:].decode('utf-8', 'replace')
-    errorclass = error_map.get(errno, InternalError)
+    errorclass = error_map.get(errno)
+    if errorclass is None:
+        errorclass = InternalError if errno < 1000 else OperationalError
     raise errorclass(errno, errval)