diff --git a/little_boxes/activitypub.py b/little_boxes/activitypub.py
index 415db83..447e0e5 100644
--- a/little_boxes/activitypub.py
+++ b/little_boxes/activitypub.py
@@ -37,7 +37,11 @@ AS_PUBLIC = "https://www.w3.org/ns/activitystreams#Public"
 COLLECTION_CTX = [
     "https://www.w3.org/ns/activitystreams",
     "https://w3id.org/security/v1",
-    {"Hashtag": "as:Hashtag", "sensitive": "as:sensitive"},
+    {
+        "Hashtag": "as:Hashtag",
+        "sensitive": "as:sensitive",
+        "toot": "http://joinmastodon.org/ns#",
+    },
 ]
 
 # Will be used to keep track of all the defined activities
diff --git a/little_boxes/backend.py b/little_boxes/backend.py
index e9c0d5a..818413b 100644
--- a/little_boxes/backend.py
+++ b/little_boxes/backend.py
@@ -14,6 +14,7 @@ from .__version__ import __version__
 from .collection import parse_collection
 from .errors import ActivityGoneError
 from .errors import ActivityNotFoundError
+from .errors import NotAnActivityError
 from .errors import ActivityUnavailableError
 from .urlutils import URLLookupFailedError
 from .urlutils import check_url as check_url
@@ -75,6 +76,9 @@ class Backend(abc.ABC):
         pass  # pragma: no cover
 
     def fetch_iri(self, iri: str, **kwargs) -> "ap.ObjectType":  # pragma: no cover
+        if not iri.startswith('http'):
+            raise NotAnActivityError(f"{iri} is not a valid IRI")
+
         try:
             self.check_url(iri)
         except URLLookupFailedError:
@@ -111,9 +115,9 @@ class Backend(abc.ABC):
 
         try:
             out = resp.json()
-        except json.JSONDecodeError:
+        except (json.JSONDecodeError, ValueError):
             # TODO(tsileo): a special error type?
-            raise ActivityUnavailableError(f"{iri} is not JSON")
+            raise NotAnActivityError(f"{iri} is not JSON")
 
         return out
 
diff --git a/little_boxes/errors.py b/little_boxes/errors.py
index 19a0bc4..935ea57 100644
--- a/little_boxes/errors.py
+++ b/little_boxes/errors.py
@@ -83,3 +83,10 @@ class ActivityUnavailableError(ServerError):
     """Raises when fetching a remote activity times out."""
 
     status_code = 503
+
+
+class NotAnActivityError(ServerError):
+    """Raised when no JSON can be decoded.
+
+    Most likely raised when stumbling upon a OStatus notice or failed lookup.
+    """