diff --git a/pytato/array.py b/pytato/array.py
index be55c4ecbd00cea69749bdb56738aae23a228b04..1b5406f163b444070f0eaf1435e4a899f29470eb 100644
--- a/pytato/array.py
+++ b/pytato/array.py
@@ -684,6 +684,10 @@ class Array:
                 indices = tuple(var(f"_{i}") for i in range(self.ndim))
                 return val[indices]
 
+        if isinstance(other, Array):
+            if self.namespace is not other.namespace:
+                raise ValueError("Operands must belong to the same namespace.")
+
         if isinstance(other, Number):
             first_expr = add_indices(var("_in0"))
             second_expr = other
@@ -931,7 +935,7 @@ class IndexLambda(_SuppliedShapeAndDtypeMixin, Array):
         else:
             return False
 
-        if index != tuple(var("_%d") % i for i in range(len(self.shape))):
+        if index != tuple(var("_%d" % i) for i in range(len(self.shape))):
             return False
 
         try: