diff --git a/gsrp_tdoa_hyperres.py b/gsrp_tdoa_hyperres.py
index 8ae9549dfe370095d3a41b3d14e012c7a8d94636..8a67ec2ff7536fe3a42405dc4458099a8b2dae9a 100755
--- a/gsrp_tdoa_hyperres.py
+++ b/gsrp_tdoa_hyperres.py
@@ -234,6 +234,7 @@ def main(args):
     result1[:, 3:] /= sr if args.temporal else sr/args.decimate
     columns = ','.join(['pos', 'db_norm', 'db'] + [f't{i}{j}'for i, j in combinations(range(sound.shape[1]), 2)])
     stem, ext = os.path.splitext(args.outfile)
+    ext = ext[1:].lower()
     if ext == 'npy':
         np.save(args.outfile, result1)
         if not args.no_hyperres: