diff --git a/telethon/sessions/abstract.py b/telethon/sessions/abstract.py
index d40d0e32..7265b6a3 100644
--- a/telethon/sessions/abstract.py
+++ b/telethon/sessions/abstract.py
@@ -20,6 +20,14 @@ class Session(ABC):
         """
         raise NotImplementedError
 
+    @property
+    @abstractmethod
+    def dc_id(self):
+        """
+        Returns the currently-used data center ID.
+        """
+        raise NotImplementedError
+
     @property
     @abstractmethod
     def server_address(self):
diff --git a/telethon/sessions/string.py b/telethon/sessions/string.py
index 1b19a2c0..fb971d82 100644
--- a/telethon/sessions/string.py
+++ b/telethon/sessions/string.py
@@ -2,6 +2,7 @@ import base64
 import ipaddress
 import struct
 
+from .abstract import Session
 from .memory import MemorySession
 from ..crypto import AuthKey
 
@@ -34,7 +35,7 @@ class StringSession(MemorySession):
             string = string[1:]
             ip_len = 4 if len(string) == 352 else 16
             self._dc_id, ip, self._port, key = struct.unpack(
-                _STRUCT_PREFORMAT.format(ip_len), self.decode(string))
+                _STRUCT_PREFORMAT.format(ip_len), StringSession.decode(string))
 
             self._server_address = ipaddress.ip_address(ip).compressed
             if any(key):
@@ -48,15 +49,15 @@ class StringSession(MemorySession):
     def decode(x: str) -> bytes:
         return base64.urlsafe_b64decode(x)
 
-    def save(self):
-        if not self._auth_key:
+    def save(self: Session):
+        if not self.auth_key:
             return ''
 
-        ip = ipaddress.ip_address(self._server_address).packed
-        return CURRENT_VERSION + self.encode(struct.pack(
+        ip = ipaddress.ip_address(self.server_address).packed
+        return CURRENT_VERSION + StringSession.encode(struct.pack(
             _STRUCT_PREFORMAT.format(len(ip)),
-            self._dc_id,
+            self.dc_id,
             ip,
-            self._port,
-            self._auth_key.key
+            self.port,
+            self.auth_key.key
         ))
diff --git a/telethon/version.py b/telethon/version.py
index 57ed6351..325308ba 100644
--- a/telethon/version.py
+++ b/telethon/version.py
@@ -1,3 +1,3 @@
 # Versions should comply with PEP440.
 # This line is parsed in setup.py:
-__version__ = '1.7.6'
+__version__ = '1.7.7'