Error converting value "null" to type 'System.Decimal'. Path 'user.id', line 1, position 10.: what it means and how to fix it

What the error means

Error converting value "null" to type 'System.Decimal'. Path 'user.id', line 1, position 10. usually means the JSON was parsed, but the value at some path does not match the type/schema your code expects (for example: array vs object, number vs string, null vs non-nullable).

Most common real-world causes

• Your code expects a different type (number vs string, bool vs string, object vs array).
• A field is null/missing but your type is non-nullable (or your code assumes it is always present).
• The API response shape changed (or differs by environment / feature flags).
• You are deserializing the wrong endpoint or the response is an error payload with a different schema.
• Date/UUID/IP formats don't match what the deserializer expects.

Fast debugging steps

• Parse into a generic JSON type first (Value/JToken/JsonElement/JsonNode) and inspect the actual shape at the failing path.
• Confirm whether the field is an array vs object and whether scalars are strings vs numbers.
• Add defensive handling for null/missing fields (optional types, defaults, or custom converters).
• Log the raw response (or a redacted sample) and validate it locally to rule out truncation or hidden prefixes.
• Update DTO/types to match the real response shape (or use a more flexible model for unstable fields).

Code example (csharp)

using System;
using Newtonsoft.Json.Linq;

// Parse to JToken first to inspect type mismatches quickly
var token = JToken.Parse(text);
Console.WriteLine(token.Type);

// Then deserialize into your target type once the shape is confirmed
// var model = token.ToObject<MyDto>();

Fix without uploading data

If the JSON contains sensitive data, validate and fix it locally. No Upload Tools runs 100% in your browser.

• JSON Validator to pinpoint the exact syntax error.
• JSON Repair to remove comments/trailing commas when the source is not strict.
• JSON Formatter to pretty-print and inspect structure.
• JSON String Escape if the issue is inside a string (newlines/tabs/quotes).

Workflow: validate -> fix the first error -> validate again -> export/convert.

FAQ

Does this mean the JSON is invalid? Not necessarily. Many of these errors happen when the JSON is valid, but your code expects a different type/shape.

What is the fastest fix? Parse into a generic JSON type first (Value/JToken/JsonElement/JsonNode), inspect the failing path, then update your DTO/type (or add a converter) to match the real response.