diff --git a/sumpy/qbx.py b/sumpy/qbx.py
index 73f7ec3d12e818cd2b111332d8285a930f306d3e..554771e7ad2b09e29de961610f7113d2d9f38e94 100644
--- a/sumpy/qbx.py
+++ b/sumpy/qbx.py
@@ -386,6 +386,17 @@ class LayerPotentialMatrixBlockGenerator(LayerPotentialBase):
 
     def __call__(self, queue, targets, sources, centers, expansion_radii,
                  index_set, **kwargs):
+        """
+        :arg targets: target point coordinates.
+        :arg sources: source point coordinates.
+        :arg centers: QBX target expansion centers.
+        :arg expansion_radii: radii for each expansion center.
+        :arg index_set: a :class:`sumpy.tools.MatrixBlockIndex` object used
+        to define the various blocks.
+        :return: a tuple of one-dimensional arrays of kernel evaluations at
+        target-source pairs described by `index_set`.
+        """
+
         knl = self.get_cached_optimized_kernel()
 
         tgtindices, srcindices = index_set.linear_indices()