diff --git a/pysaliency/datasets/fixations.py b/pysaliency/datasets/fixations.py
index 156d48a..01a02a4 100644
--- a/pysaliency/datasets/fixations.py
+++ b/pysaliency/datasets/fixations.py
@@ -278,7 +278,7 @@ def y_int(self):
 
     @property
     def subject_count(self):
-        return self.subject.max()+1
+        return int(self.subject.max())+1
 
     def copy(self):
         cfix = Fixations(self.x.copy(), self.y.copy(), self.t.copy(),