diff --git a/src/labthings_fastapi/actions/__init__.py b/src/labthings_fastapi/actions/__init__.py
index 3cd6324e..5d51f84a 100644
--- a/src/labthings_fastapi/actions/__init__.py
+++ b/src/labthings_fastapi/actions/__init__.py
@@ -35,7 +35,7 @@
     InvocationCancelledError,
     invocation_logger,
 )
-from ..outputs.blob import BlobIOContextDep
+from ..outputs.blob import BlobIOContextDep, blobdata_to_url_ctx
 
 if TYPE_CHECKING:
     # We only need these imports for type hints, so this avoids circular imports.
@@ -46,6 +46,14 @@
 """The API route used to list `.Invocation` objects."""
 
 
+class NoBlobManagerError(RuntimeError):
+    """Raised if an API route accesses Invocation outputs without a BlobIOContextDep.
+
+    Any access to an invocation output must have BlobIOContextDep as a dependency, as
+    the output may be a blob, and the blob needs this context to resolve its URL.
+    """
+
+
 class Invocation(Thread):
     """A Thread subclass that retains output values and tracks progress.
 
@@ -123,7 +131,23 @@ def id(self) -> uuid.UUID:
 
     @property
     def output(self) -> Any:
-        """Return value of the Action. If the Action is still running, returns None."""
+        """Return value of the Action. If the Action is still running, returns None.
+
+        :raise NoBlobManagerError: If this is called in a context where the blob
+            manager context variables are not available. This stops errors being raised
+            later once the blob is returned and tries to serialise. If the errors
+            happen during serialisation the stack-trace will not clearly identify
+            the route with the missing dependency.
+        """
+        try:
+            blobdata_to_url_ctx.get()
+        except LookupError as e:
+            raise NoBlobManagerError(
+                "An invocation output has been requested from a api route that "
+                "doesn't have a BlobIOContextDep dependency. This dependency is needed "
+                " for blobs to identify their url."
+            ) from e
+
         with self._status_lock:
             return self._return_value
 
diff --git a/src/labthings_fastapi/descriptors/action.py b/src/labthings_fastapi/descriptors/action.py
index 58ed321c..6129775f 100644
--- a/src/labthings_fastapi/descriptors/action.py
+++ b/src/labthings_fastapi/descriptors/action.py
@@ -339,7 +339,9 @@ def start_action(
             ),
             summary=f"All invocations of {self.name}.",
         )
-        def list_invocations(action_manager: ActionManagerContextDep):
+        def list_invocations(
+            action_manager: ActionManagerContextDep, _blob_manager: BlobIOContextDep
+        ):
             return action_manager.list_invocations(self, thing)
 
     def action_affordance(
diff --git a/tests/test_actions.py b/tests/test_actions.py
index 26d4edb7..18a5d257 100644
--- a/tests/test_actions.py
+++ b/tests/test_actions.py
@@ -18,6 +18,30 @@ def run(payload=None):
     return run
 
 
+def test_get_action_invocations():
+    """Test that running "get" on an action returns a list of invocations."""
+    with TestClient(server.app) as client:
+        # When we start the action has no invocations
+        invocations_before = client.get("/thing/increment_counter").json()
+        assert invocations_before == []
+        # Start the action
+        r = client.post("/thing/increment_counter")
+        assert r.status_code in (200, 201)
+        # Now it is started, there is a list of 1 dictionary containing the
+        # invocation information.
+        invocations_after = client.get("/thing/increment_counter").json()
+        assert len(invocations_after) == 1
+        assert isinstance(invocations_after, list)
+        assert isinstance(invocations_after[0], dict)
+        assert "status" in invocations_after[0]
+        assert "id" in invocations_after[0]
+        assert "action" in invocations_after[0]
+        assert "href" in invocations_after[0]
+        assert "timeStarted" in invocations_after[0]
+        # Let the task finish before ending the test
+        poll_task(client, r.json())
+
+
 def test_counter():
     with TestClient(server.app) as client:
         before_value = client.get("/thing/counter").json()