go-playground/validator · error

Bad field name %s

Error message

Bad field name %s

What it means

Panics inside the isUnique validator's slice-of-struct branch when the parameter naming the key field does not exist on the struct elements. After resolving each element to a struct, FieldByName(param) returns an invalid value because the tag parameter names a field that is not present on that struct; the guard panics to signal a misconfigured unique=FieldName tag rather than silently passing.

Source

Thrown at baked_in.go:427

			if elem.Kind() == reflect.Ptr {
				if elem.IsNil() {
					if _, ok := seen[nilKey]; ok {
						return false
					}
					seen[nilKey] = struct{}{}
					continue
				}
				elem = elem.Elem()
			}

			if elem.Kind() != reflect.Struct {
				panic(fmt.Sprintf("Bad field type %s", elem.Type()))
			}

			sf := elem.FieldByName(param)
			if !sf.IsValid() {
				panic(fmt.Sprintf("Bad field name %s", param))
			}

			var key interface{}

			if sf.Kind() == reflect.Ptr {
				if sf.IsNil() {
					key = nilKey
				} else {
					key = sf.Elem().Interface()
				}
			} else {
				key = sf.Interface()
			}

			if _, ok := seen[key]; ok {
				return false
			}
			seen[key] = struct{}{}

View on GitHub (pinned to facf128d2e)

Solutions

1. Correct the unique tag parameter to match an actual field name on the struct (case-sensitive)
2. Add the missing key field to the struct if it was intended
3. Keep the field name in sync when renaming struct fields used by unique tags

When it happens

Trigger: Thrown at baked_in.go:427 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of go-playground/validator@facf128d2e (2026-09-02). Data as JSON: /api/errors/0de22ccce116f3de. Report an issue: GitHub.