diff --git a/sumpy/tools.py b/sumpy/tools.py
index d37ef2aa8f98337febebf20320cd91aaec78f6fa..616bd824c1614064a000f945b69c1400c42804c4 100644
--- a/sumpy/tools.py
+++ b/sumpy/tools.py
@@ -437,19 +437,19 @@ def diff_transformation(derivative_transformation, variable_idx, variables):
     variable given by **variable_idx** and return a new derivative transformation
     dictionary
     """
-    new_transformation = defaultdict(lambda: 0)
+    new_derivative_coeff_dict = defaultdict(lambda: 0)
     for mi, coeff in derivative_transformation.items():
         # In the case where we have x * u.diff(x), the result should
         # be x.diff(x) + x * u.diff(x, x)
         # Calculate the first term by differentiating the coefficients
         new_coeff = sym.sympify(coeff).diff(variables[variable_idx])
         if new_coeff != 0:
-            new_transformation[mi] += new_coeff
+            new_derivative_coeff_dict[mi] += new_coeff
         # Next calculate the second term by differentitating the derivatives
         new_mi = list(mi)
         new_mi[variable_idx] += 1
-        new_transformation[tuple(new_mi)] += coeff
-    return dict(new_transformation)
+        new_derivative_coeff_dict[tuple(new_mi)] += coeff
+    return dict(new_derivative_coeff_dict)
 
 # }}}