Domain pattern '%s' contains empty label at position %d

Error message

Domain pattern '%s' contains empty label at position %d

What it means

Panics from domain.go:88 when a dot-separated label is empty, i.e. consecutive dots in the pattern ("example..com"). Empty labels are invalid per RFC 1035 and would produce ambiguous matching, so parseDomainPattern rejects them per-label.

Source

Thrown at domain.go:88

	}

	parts := strings.Split(pattern, ".")

	// Prevent DoS from patterns with excessive label counts
	if len(parts) > maxDomainParts {
		panic(fmt.Sprintf("Domain pattern '%s' has %d parts, which exceeds the maximum of %d",
			pattern, len(parts), maxDomainParts))
	}

	m := domainMatcher{
		parts:    make([]string, len(parts)),
		numParts: len(parts),
	}

	for i, part := range parts {
		// Validate no empty labels (e.g., "example..com" is invalid)
		if part == "" {
			panic(fmt.Sprintf("Domain pattern '%s' contains empty label at position %d", pattern, i))
		}

		if part[0] == ':' {
			// Validate parameter name is not empty
			if len(part) == 1 {
				panic(fmt.Sprintf("Domain pattern '%s' contains empty parameter name at position %d", pattern, i))
			}
			paramName := part[1:]
			// Validate parameter name contains only ASCII-safe characters (a-z, A-Z, 0-9, underscore, hyphen).
			// Using explicit ASCII ranges rather than unicode.IsLetter/IsDigit to reject non-ASCII
			// characters that are invalid in DNS names.
			for _, ch := range paramName {
				if !isASCIIAlphanumeric(ch) && ch != '_' && ch != '-' {
					panic(fmt.Sprintf("Domain pattern '%s' contains invalid parameter name '%s' with character '%c'", pattern, paramName, ch))
				}
			}
			m.paramIdx = append(m.paramIdx, i)
			m.paramNames = append(m.paramNames, paramName) // preserve original case

View on GitHub (pinned to 9a4c7e57fe)

Solutions

Fix the pattern to have no empty labels: "example.com", "sub.example.com".
Sanitize the pattern by collapsing/stripping stray dots before calling Domain().
Build patterns from a slice of non-empty labels joined by ".".

Example fix

// before
domain := tenant + "." + suffix // suffix == "" -> "acme."
app.Domain(domain).Get("/", h)

// after
labels := []string{tenant}
if suffix != "" {
    labels = append(labels, suffix)
}
domain := strings.Join(labels, ".") // "acme"
app.Domain(domain).Get("/", h)

Defensive patterns

Strategy: validation

Validate before calling

for _, label := range strings.Split(pattern, ".") {
    if label == "" {
        return fmt.Errorf("domain pattern has empty label")
    }
}
app.Domain(pattern).Get("/", h)

Prevention

Build patterns by joining a slice of non-empty labels with ".".
Sanitize stray leading/trailing/double dots before registering.
Unit-test pattern construction with empty inputs.

When it happens

Trigger: app.Domain("example..com"), app.Domain(".example.com") (leading dot yields empty first label after split), or app.Domain("example.com.") is fine (trailing dot trimmed), but "example..com." is not.

Common situations: Concatenating labels with a missing value: domain := part + "." + part2 where part2 is empty; trailing/leading dots from sloppy string formatting; config typos.

Related errors

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/6b4f3f00249a5121.json. Report an issue: GitHub.