2 files changed, 22 insertions, 16 deletions
diff --git a/synapse/groups/groups_server.py b/synapse/groups/groups_server.py
index 3599bfe9cf..23beb3187e 100644
--- a/synapse/groups/groups_server.py
+++ b/synapse/groups/groups_server.py
@@ -15,7 +15,6 @@
 
 import logging
 
-from synapse import types
 from synapse.api.errors import SynapseError
 from synapse.types import GroupID, RoomID, UserID, get_domain_from_id
 from twisted.internet import defer
@@ -696,9 +695,11 @@ class GroupsServerHandler(object):
     def create_group(self, group_id, user_id, content):
         group = yield self.check_group_is_ours(group_id)
 
-        _validate_group_id(group_id)
-
         logger.info("Attempting to create group with ID: %r", group_id)
+
+        # parsing the id into a GroupID validates it.
+        group_id_obj = GroupID.from_string(group_id)
+
         if group:
             raise SynapseError(400, "Group already exists")
 
@@ -708,7 +709,7 @@ class GroupsServerHandler(object):
                 raise SynapseError(
                     403, "Only server admin can create group on this server",
                 )
-            localpart = GroupID.from_string(group_id).localpart
+            localpart = group_id_obj.localpart
             if not localpart.startswith(self.hs.config.group_creation_prefix):
                 raise SynapseError(
                     400,
@@ -784,15 +785,3 @@ def _parse_visibility_from_contents(content):
         is_public = True
 
     return is_public
-
-
-def _validate_group_id(group_id):
-    """Validates the group ID is valid for creation on this home server
-    """
-    localpart = GroupID.from_string(group_id).localpart
-
-    if types.contains_invalid_mxid_characters(localpart):
-        raise SynapseError(
-            400,
-            "Group ID can only contain characters a-z, 0-9, or '=_-./'",
-        )
diff --git a/synapse/types.py b/synapse/types.py
index 88eb818de4..5e3d1fc0b2 100644
--- a/synapse/types.py
+++ b/synapse/types.py
@@ -161,6 +161,23 @@ class GroupID(DomainSpecificString):
     """Structure representing a group ID."""
     SIGIL = "+"
 
+    @classmethod
+    def from_string(cls, s):
+        group_id = super(GroupID, cls).from_string(s)
+        if not group_id.localpart:
+            raise SynapseError(
+                400,
+                "Group ID cannot be empty",
+            )
+
+        if contains_invalid_mxid_characters(group_id.localpart):
+            raise SynapseError(
+                400,
+                "Group ID can only contain characters a-z, 0-9, or '=_-./'",
+            )
+
+        return group_id
+
 
 mxid_localpart_allowed_characters = set("_-./=" + string.ascii_lowercase + string.digits)