ALT Aha! Now we can see the exact problem. There's a BOM (Byte Order Mark) character in the CSV's "link" column name. Let's break down what we see: string The character code 65279 is a Zero Width No-Break Space (UTF-8 BOM), which is invisible but makes the strings different. That's why the property lookup fails! Here's how to fix the mapper function: route.ts , This solution: Removes the BOM character when comparing keys Handles the mapping for all fields, not just the link field Maintains the original functionality while fixing the character encoding issue The issue was coming from the CSV file being saved with a BOM marker, which is common when files are saved from certain programs (especially Excel). The fix handles this gracefully without requiring changes to the input file.