github/github-mcp-server · error

value cannot be null for field %q

Error message

value cannot be null for field %q

What it means

optionalIssueWriteFields (pkg/github/issues.go:271) rejects any issue_fields item whose value key is present but explicitly null. JSON null unmarshals to a nil any, which the validation treats as an invalid attempt to null a field — the API has no null value, only deletion. Use "delete": true or omit the key.

Source

Thrown at pkg/github/issues.go:271

		return nil, fmt.Errorf("issue_fields must be an array")
	}

	issueFields := make([]issueWriteFieldInput, 0, len(inputMaps))
	for _, itemMap := range inputMaps {
		fieldName, err := RequiredParam[string](itemMap, "field_name")
		if err != nil || strings.TrimSpace(fieldName) == "" {
			return nil, fmt.Errorf("field_name is required for each issue_fields item")
		}

		fieldOptionName, err := OptionalParam[string](itemMap, "field_option_name")
		if err != nil {
			return nil, err
		}

		deleteField, _ := OptionalParam[bool](itemMap, "delete")
		value, hasValue := itemMap["value"]
		if hasValue && value == nil {
			return nil, fmt.Errorf("value cannot be null for field %q", fieldName)
		}

		if deleteField {
			if hasValue || fieldOptionName != "" {
				return nil, fmt.Errorf("issue field %q cannot specify 'delete' together with 'value' or 'field_option_name'", fieldName)
			}
			issueFields = append(issueFields, issueWriteFieldInput{
				FieldName: fieldName,
				Delete:    true,
			})
			continue
		}

		if hasValue && fieldOptionName != "" {
			return nil, fmt.Errorf("issue field %q cannot specify both value and field_option_name", fieldName)
		}

		if !hasValue && fieldOptionName == "" {

View on GitHub (pinned to 0ea1f775a7)

Solutions

1. To clear a field use {"field_name":"Due Date","delete":true}
2. To leave a field alone, omit the value key entirely
3. Mark optional pointer fields with omitempty when marshaling Go structs to tool args

Example fix

# before
{"issue_fields": [{"field_name": "Due Date", "value": null}]}

# after
{"issue_fields": [{"field_name": "Due Date", "delete": true}]}

for _, item := range inputMaps {
    if v, present := item["value"]; present && v == nil {
        return errors.New("value cannot be null; use delete:true or omit the key")
    }
}

func hasNullValue(item map[string]any) bool {
    v, present := item["value"]
    return present && v == nil
}

When it happens

Trigger: update_issue with {"field_name":"Due Date","value":null}; common when callers try to clear a field by nulling it, or when JSON builders emit null for absent optional members.

Common situations: LLM tool args synthesizing null for optional fields; client code marshaling nil pointers as null instead of omitempty; users assuming null clears a field.

Related errors

• each issue_fields item must be an object
• issue_fields must be an array
• field_name is required for each issue_fields item
• issue field %q cannot specify 'delete' together with 'value'
• issue field %q cannot specify both value and field_option_na

AI-assisted analysis of github/github-mcp-server@0ea1f775a7 (2026-08-15). Data as JSON: /api/errors/6cf63f4208ae2bdc. Report an issue: GitHub.