diff --git a/src/tinygp/kernels/quasisep.py b/src/tinygp/kernels/quasisep.py
index b380f08..2ed03f6 100644
--- a/src/tinygp/kernels/quasisep.py
+++ b/src/tinygp/kernels/quasisep.py
@@ -243,9 +243,13 @@ def coord_to_sortable(self, X: JAXArray) -> JAXArray:
         return self.kernel1.coord_to_sortable(X)
 
     def _block_or_dense(self, m1: JAXArray, m2: JAXArray) -> JAXArray:
-        if self.use_block:
-            return Block(m1, m2)
-        return jsp_block_diag(m1, m2)
+        if not self.use_block:
+            return jsp_block_diag(m1, m2)
+
+        # Ensure we don't nest Block objects to fix Issue #265
+        blocks1 = m1.blocks if isinstance(m1, Block) else (m1,)
+        blocks2 = m2.blocks if isinstance(m2, Block) else (m2,)
+        return Block(*blocks1, *blocks2)
 
     def design_matrix(self) -> JAXArray:
         return self._block_or_dense(