diff --git a/src/postprocessor/grouper.py b/src/postprocessor/grouper.py
index 4903957216f0e6a8609a3af2e9f899494fe1a4f8..d3699c420dfe908594ac65941c1996808633c0a0 100644
--- a/src/postprocessor/grouper.py
+++ b/src/postprocessor/grouper.py
@@ -49,7 +49,7 @@ class Grouper(ABC):
     def tinterval(self) -> float:
         """Find the time interval for all positions."""
         tintervals = list(
-            set([s.tinterval / 60 for s in self.positions.values()])
+            np.unique([s.tinterval / 60 for s in self.positions.values()])
         )
         assert (
             len(tintervals) == 1