diff --git a/telethon/sessions/memory.py b/telethon/sessions/memory.py
index 43ddde4b..e5223cac 100644
--- a/telethon/sessions/memory.py
+++ b/telethon/sessions/memory.py
@@ -177,7 +177,7 @@ class MemorySession(Session):
                 key = utils.get_peer_id(key)
                 exact = True
             else:
-                exact = False
+                exact = not isinstance(key, int) or key < 0
 
         result = None
         if isinstance(key, str):
diff --git a/telethon/telegram_client.py b/telethon/telegram_client.py
index 937540da..e35b996c 100644
--- a/telethon/telegram_client.py
+++ b/telethon/telegram_client.py
@@ -2285,9 +2285,10 @@ class TelegramClient(TelegramBareClient):
                 'Cannot turn "{}" into an input entity.'.format(original_peer)
             )
 
-        # Add the mark to the peers if the user passed a Peer (not an int)
+        # Add the mark to the peers if the user passed a Peer (not an int),
+        # or said ID is negative. If it's negative it's been marked already.
         # Look in the dialogs with the hope to find it.
-        mark = not isinstance(peer, int)
+        mark = not isinstance(peer, int) or peer < 0
         target_id = utils.get_peer_id(peer)
         req = GetDialogsRequest(
             offset_date=None,