json: cannot unmarshal null into Go value of type int64: what it means and how to fix it
TL;DR: Validate locally, fix the first real error, validate again (no upload).
Troubleshoot json: cannot unmarshal null into Go value of type int64 quickly and validate JSON locally (no upload).
What the error means
json: cannot unmarshal null into Go value of type int64 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 (go)
// 1) Decode into interface{} / map first to inspect real types
var any interface{}
if err := json.Unmarshal(body, &any); err != nil {
return err
}
// 2) Inspect suspicious keys/fields (example)
m, _ := any.(map[string]interface{})
for k := range m {
fmt.Println("key:", k)
break // print a few keys first
}
// 3) Only then unmarshal into a strong type
// var dto MyDto
// dec := json.NewDecoder(bytes.NewReader(body))
// dec.DisallowUnknownFields() // if you want strict schema
// if err := dec.Decode(&dto); err != nil { return err }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.
Related tools
Related guides
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Quick fix checklist
- Reproduce the error on a minimal input.
- Check type/format and field mapping.
- Apply the smallest safe fix.
- Validate on production-like payload.
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