diff --git a/sumpy/tools.py b/sumpy/tools.py
index 88ff5241799c210a522e8b54935ff0d1ae9dde47..a18358c939e9932340058487c6af20efc4951fe3 100644
--- a/sumpy/tools.py
+++ b/sumpy/tools.py
@@ -65,20 +65,23 @@ class MiDerivativeTaker(object):
         return np.array(a, dtype=int) - np.array(b, dtype=int)
 
     def diff(self, mi):
-        closest_mi = min(
-            (other_mi
-                for other_mi in self.cache_by_mi.keys()
-                if (np.array(mi) >= np.array(other_mi)).all()),
-            key=lambda other_mi: sum(self.mi_dist(mi, other_mi)))
-
-        expr = self.cache_by_mi[closest_mi]
-        current_mi = np.array(closest_mi, dtype=int)
-        for idx, (mi_i, vec_i) in enumerate(
-                    zip(self.mi_dist(mi, closest_mi), self.var_list)):
-            for i in range(1, 1 + mi_i):
-                current_mi[idx] += 1
-                expr = expr.diff(vec_i)
-                self.cache_by_mi[tuple(current_mi)] = expr
+        try:
+            expr = self.cache_by_mi[mi]
+        except KeyError:
+            closest_mi = min(
+                (other_mi
+                    for other_mi in self.cache_by_mi.keys()
+                    if (np.array(mi) >= np.array(other_mi)).all()),
+                key=lambda other_mi: sum(self.mi_dist(mi, other_mi)))
+
+            expr = self.cache_by_mi[closest_mi]
+            current_mi = np.array(closest_mi, dtype=int)
+            for idx, (mi_i, vec_i) in enumerate(
+                        zip(self.mi_dist(mi, closest_mi), self.var_list)):
+                for i in range(1, 1 + mi_i):
+                    current_mi[idx] += 1
+                    expr = expr.diff(vec_i)
+                    self.cache_by_mi[tuple(current_mi)] = expr
 
         return expr