diff --git a/xarray/core/utils.py b/xarray/core/utils.py
index 6ed0b2c4318..7270a8f587f 100644
--- a/xarray/core/utils.py
+++ b/xarray/core/utils.py
@@ -536,8 +536,7 @@ def discard(self, value: T) -> None:
     # Additional methods
 
     def update(self, values: Iterable[T]) -> None:
-        for v in values:
-            self._d[v] = None
+        self._d.update(dict.fromkeys(values))
 
     def __repr__(self) -> str:
         return f"{type(self).__name__}({list(self)!r})"