1 files changed, 1 insertions, 1 deletions
diff --git a/synapse/types.py b/synapse/types.py
index 9cffc33d27..8c51e00e8a 100644
--- a/synapse/types.py
+++ b/synapse/types.py
@@ -47,7 +47,7 @@ class DomainSpecificString(
     @classmethod
     def from_string(cls, s):
         """Parse the string given by 's' into a structure object."""
-        if s[0] != cls.SIGIL:
+        if len(s) < 1 or s[0] != cls.SIGIL:
             raise SynapseError(400, "Expected %s string to start with '%s'" % (
                 cls.__name__, cls.SIGIL,
             ))