diff --git a/telethon/errors/__init__.py b/telethon/errors/__init__.py
index ef327041..af1e089c 100644
--- a/telethon/errors/__init__.py
+++ b/telethon/errors/__init__.py
@@ -34,7 +34,13 @@ def rpc_message_to_error(rpc_error, request):
             capture = int(m.group(1)) if m.groups() else None
             return cls(request, capture=capture)
 
-    cls = base_errors.get(rpc_error.error_code)
+    # Some errors are negative:
+    # * -500 for "No workers running",
+    # * -503 for "Timeout"
+    #
+    # We treat them as if they were positive, so -500 will be treated
+    # as a `ServerError`, etc.
+    cls = base_errors.get(abs(rpc_error.error_code))
     if cls:
         return cls(request, rpc_error.error_message)
 
diff --git a/telethon/errors/rpcbaseerrors.py b/telethon/errors/rpcbaseerrors.py
index edbab5ba..fcbbc8b3 100644
--- a/telethon/errors/rpcbaseerrors.py
+++ b/telethon/errors/rpcbaseerrors.py
@@ -100,7 +100,7 @@ class ServerError(RPCError):
     for example, there was a disruption while accessing a database or file
     storage.
     """
-    code = 500
+    code = 500  # Also witnessed as -500
     message = 'INTERNAL'
 
     def __init__(self, request, message):
@@ -113,7 +113,7 @@ class BotTimeout(RPCError):
     Clicking the inline buttons of bots that never (or take to long to)
     call ``answerCallbackQuery`` will result in this "special" RPCError.
     """
-    code = -503
+    code = 503  # Only witnessed as -503
     message = 'Timeout'
 
     def __init__(self, request, message):