diff --git a/src/MatchesWebApp.py b/src/MatchesWebApp.py
index a69a194..2674998 100644
--- a/src/MatchesWebApp.py
+++ b/src/MatchesWebApp.py
@@ -454,6 +454,8 @@ class MatchesWebApp:
                 return Response("build already running", status=409)
             try:
                 result = self._mover.build_matches_only()
+            except Exception as exc:
+                return Response(f"build failed: {exc}", status=500)
             finally:
                 self._build_lock.release()
             return jsonify(result)