What causes "invalid type %T for map[string]string" in docker/cli?

Provide a mapping: `sysctls: { net.core.somaxconn: 1024 }`.

invalid type %T for map[string]string

Error message

invalid type %T for map[string]string

What it means

Thrown by transformMapStringString, the transformer for fields like `sysctls`, `labels`, `extra_hosts` (mapping form). The value must be a `map[string]any` (YAML mapping) or `map[string]string`; any other shape (scalar, sequence) is rejected.

Solutions

Provide a mapping: `sysctls: { net.core.somaxconn: 1024 }`.
For list-style fields use the list form only where supported (some fields accept both via a different transformer).

Example fix

# before
web:
  sysctls: net.core.somaxconn
# after
web:
  sysctls:
    net.core.somaxconn: "1024"

Defensive patterns

Strategy: type-guard

Validate before calling

func validateMapStringString(field string, v any) error {
    switch v.(type) {
    case map[string]any, map[string]string:
        return nil
    default:
        return fmt.Errorf("%s must be a mapping, got %T", field, v)
    }
}

Type guard

func isStringMapping(v any) bool {
    switch v.(type) {
    case map[string]any, map[string]string:
        return true
    }
    return false
}

Prevention

Remember `sysctls`/`labels`/`extra_hosts`(map form) need key:value pairs.
Use `docker compose config` to render and check.
Watch indentation: a mapping must be more indented than its parent key.

When it happens

Trigger: A field expecting a mapping is given a scalar or a list — e.g. `sysctls: net.core.somaxconn` (a single string) or `labels: [a,b]`. The transformer is invoked per-key during Transform().

Common situations: Forgetting the value side of a mapping; using a list where a map is required; typo in indentation.

Related errors

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/a44b03dc46d65b10. Report an issue: GitHub.

Appendix: source

Thrown at cli/compose/loader/loader.go:692

	return obj, nil
}

func absPath(workingDir string, filePath string) string {
	if filepath.IsAbs(filePath) {
		return filePath
	}
	return filepath.Join(workingDir, filePath)
}

var transformMapStringString TransformerFunc = func(data any) (any, error) {
	switch value := data.(type) {
	case map[string]any:
		return toMapStringString(value, false), nil
	case map[string]string:
		return value, nil
	default:
		return data, fmt.Errorf("invalid type %T for map[string]string", value)
	}
}

var transformExternal TransformerFunc = func(data any) (any, error) {
	switch value := data.(type) {
	case bool:
		return map[string]any{"external": value}, nil
	case map[string]any:
		return map[string]any{"external": true, "name": value["name"]}, nil
	default:
		return data, fmt.Errorf("invalid type %T for external", value)
	}
}

var transformServicePort TransformerFunc = func(data any) (any, error) {
	switch entries := data.(type) {
	case []any:
		// We process the list instead of individual items here.

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