diff --git a/modules/programs/zsh/prezto.nix b/modules/programs/zsh/prezto.nix
index 1aa3af1e..9a94f238 100644
--- a/modules/programs/zsh/prezto.nix
+++ b/modules/programs/zsh/prezto.nix
@@ -16,7 +16,8 @@ let
 
       caseSensitive = mkOption {
         type = types.nullOr types.bool;
-        default = null;
+        # See <https://github.com/nix-community/home-manager/issues/2255>.
+        default = true;
         example = true;
         description =
           "Set case-sensitivity for completion, history lookup, etc.";