diff --git a/sumpy/kernel.py b/sumpy/kernel.py
index e7864ebe6872300885f0bdc48c5a6de41733f234..501153606750e424f4effb82d20fa7121a6c9997 100644
--- a/sumpy/kernel.py
+++ b/sumpy/kernel.py
@@ -544,7 +544,7 @@ class StressletKernel(ExpressionKernel):
     def update_persistent_hash(self, key_hash, key_builder):
         key_hash.update(type(self).__name__.encode())
         key_builder.rec(key_hash, (
-            self.dim, self.icomp, self.jcomp, self.kcomp, self.viscosity_mu_name,
+            self.dim, self.icomp, self.jcomp, self.kcomp,
             self.viscosity_mu_name))
 
     def __repr__(self):