diff --git a/sweep_layouts.py b/sweep_layouts.py
index 159ea49..fc6f754 100644
--- a/sweep_layouts.py
+++ b/sweep_layouts.py
@@ -141,6 +141,10 @@ def main() -> None:
 
     rows = []
     raw_json_lines = []
+    invalid_fanouts = [f for f in args.fanouts if f <= 1]
+    if invalid_fanouts:
+        print(f"ERROR: Fanouts must be > 1, got {invalid_fanouts}", file=sys.stderr)
+        sys.exit(1)
 
     for leaves in leaf_counts:
         for fanout in args.fanouts: