diff --git a/sumpy/kernel.py b/sumpy/kernel.py
index 039368916020ccfc4cb6f5a4f20bb92be62a5fa1..efb4b09398f40bb0ad87e54bebc250ae877b254c 100644
--- a/sumpy/kernel.py
+++ b/sumpy/kernel.py
@@ -238,6 +238,9 @@ class Kernel:
 
         The typical use of this function is to apply target-variable
         derivatives to the kernel.
+        
+        :arg expr: may be a :class:`sympy.Expr` or a
+            :class:`sumpy.tools.DifferentiatedExprDerivativeTaker`.
         """
         return expr