diff --git a/.circleci/config.yml b/.circleci/config.yml
index 500bc405295e4c8862c3062099a4a939e4b54e2b..6efdb402b7acf9ab44ebe55acaaa6b64d0e0b71e 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -144,8 +144,11 @@ jobs:
         type: "string"
 
     docker:
-      # Run in a highly Nix-capable environment.
-      - image: "nixos/nix:latest"
+      # Run in a highly Nix-capable environment.  We used to use `latest` but
+      # 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"
 
     # Tahoe-LAFS requires more memory than we get from the default resource
     # class and sometimes we have to build it.