diff --git a/.circleci/config.yml b/.circleci/config.yml
index 6efdb402b7acf9ab44ebe55acaaa6b64d0e0b71e..dc041b7bfd568d2d91a2a5f7b484061c7029842a 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -148,7 +148,7 @@ jobs:
       # one day someone pushed a bad revision to it and our CI broke.  So now
       # we just pin some recent version.  Who would have thought a floating
       # dependency would cause build instability?
-      - image: "nixos/nix:2.3"
+      - image: "nixos/nix:2.3.16"
 
     # Tahoe-LAFS requires more memory than we get from the default resource
     # class and sometimes we have to build it.