What causes "mismatched types nil and %s" in go-delve/delve?

Compare against the zero value instead: x == 0 for ints, x == "" for strings, x == false for bools

mismatched types nil and %s

Error message

mismatched types nil and %s

What it means

Raised by the type-conversion logic (convertToType) when the source expression is untyped nil (nilVariable). nil is only assignable to slice, map, func, pointer, chan, and interface types in Go. If the target type typ is anything else (int, string, struct, bool...), Delve reports 'mismatched types nil and <target>'.

Source

Thrown at pkg/proc/eval.go:2781

func (v *Variable) isType(typ godwarf.Type, kind reflect.Kind) error {
	if v.DwarfType != nil {
		if typ == nil || !sameType(typ, v.RealType) {
			return &typeConvErr{v.DwarfType, typ}
		}
		return nil
	}

	if typ == nil {
		return nil
	}

	if v == nilVariable {
		switch kind {
		case reflect.Slice, reflect.Map, reflect.Func, reflect.Ptr, reflect.Chan, reflect.Interface:
			return nil
		default:
			return fmt.Errorf("mismatched types nil and %s", typ.String())
		}
	}

	converr := fmt.Errorf("can not convert %s constant to %s", v.Value, typ.String())

	if v.Value == nil {
		return converr
	}

	switch typ.(type) {
	case *godwarf.IntType:
		if v.Value.Kind() != constant.Int {
			return converr
		}
	case *godwarf.UintType:
		if v.Value.Kind() != constant.Int {
			return converr
		}

View on GitHub (pinned to a23773e6c3)

Solutions

Compare against the zero value instead: x == 0 for ints, x == "" for strings, x == false for bools
Only assign/convert nil to reference types: *T, map, slice, chan, func, interface
If checking for emptiness of a struct, test a field or use a pointer to the struct and compare to nil
Remove the invalid nil cast from breakpoint conditions/watch expressions

Example fix

// before
cond: x == nil            // x is int
// after
cond: x == 0              // zero-value comparison for int

Defensive patterns

Strategy: validation

Validate before calling

// nil is only valid for slice/map/func/ptr/chan/interface
// for other types compare to zero value:
cond: x == 0   // int
cond: s == ""  // string

Type guard

// only use nil when the target type is a reference type
// func isNilable(typ string) bool { return strings.Contains(typ, "*") || typ=="map" || typ=="slice" || typ=="chan" || typ=="func" }

Prevention

Never cast nil to value types (int, string, struct, bool)
Compare value types to their zero values instead of nil
Use pointers for structs that need nil-ability

When it happens

Trigger: Evaluating expressions like (*T)(nil) on wrong types: e.g. int(nil), string(nil), or struct-typed conversions from nil in the debugger console or breakpoint conditions, hitting the nilVariable check in convertToType.

Common situations: Typing nil where Go itself would reject it; autogenerated conditions comparing a non-reference variable to nil (e.g. myInt == nil converted via type assertion); ported code assuming nil converts to any zero value.

Related errors

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/bb6234e4353bb1c0. Report an issue: GitHub.