diff --git a/pymbolic/mapper/persistent_hash.py b/pymbolic/mapper/persistent_hash.py
index ad2deae..5f10d90 100644
--- a/pymbolic/mapper/persistent_hash.py
+++ b/pymbolic/mapper/persistent_hash.py
@@ -49,3 +49,9 @@ def map_constant(self, expr):
                 expr = expr.item()
 
         self.key_hash.update(repr(expr).encode("utf8"))
+
+    def map_comparison(self, expr):
+        if self.visit(expr):
+            self.rec(expr.left)
+            self.key_hash.update(repr(expr.operator).encode("utf8"))
+            self.rec(expr.right)