1 files changed, 13 insertions, 1 deletions
diff --git a/nix/readTree/default.nix b/nix/readTree/default.nix
index 0c323bbdc8..ce4c7f032a 100644
--- a/nix/readTree/default.nix
+++ b/nix/readTree/default.nix
@@ -16,6 +16,11 @@ let
     readDir
     substring;
 
+  assertMsg = pred: msg:
+    if pred
+    then true
+    else builtins.trace msg false;
+
   argsWithPath = args: parts:
     let meta.locatedAt = parts;
     in meta // (if isAttrs args then args else args meta);
@@ -38,7 +43,14 @@ let
   # The marker is added to every set that was imported directly by
   # readTree.
   importWithMark = args: path: parts:
-    let imported = import path (argsWithPath args parts);
+    let
+      importedFile = import path;
+      pathType = builtins.typeOf importedFile;
+      imported =
+        assert assertMsg
+          (pathType == "lambda")
+          "readTree: trying to import ${toString path}, but it’s a ${pathType}, you need to make it a function like { depot, pkgs, ... }";
+        importedFile (argsWithPath args parts);
     in if (isAttrs imported)
       then imported // (marker parts)
       else imported;