Domain pattern '%s' contains empty parameter name at positio

Error message

Domain pattern '%s' contains empty parameter name at position %d

What it means

Panics from domain.go:94 when a domain parameter is just the colon marker with no name, e.g. a label equal to ":". Parameter labels must be ":name"; a bare ":" has no parameter name to bind and would break DomainParam lookups.

Source

Thrown at domain.go:94

		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
			m.parts[i] = part                              // keep ":param" marker for matching
		} else {
			// Only lowercase constant labels (RFC 4343)
			// Enforce RFC 1035 per-label length limit (63 characters)
			if len(part) > 63 {
				panic(fmt.Sprintf("Domain pattern '%s' has label '%s' exceeding RFC 1035 limit of 63 characters (%d chars)",

View on GitHub (pinned to 9a4c7e57fe)

Solutions

Give every parameter a name: app.Domain(":tenant.example.com").
Validate that each ":"-prefixed label has length > 1 before calling Domain().
Default empty parameter names to a concrete name like "sub".

Example fix

// before
name := params["name"] // ""
app.Domain(":" + name + ".example.com").Get("/", h)

// after
name := params["name"]
if name == "" {
    name = "tenant"
}
app.Domain(":" + name + ".example.com").Get("/", h)

Defensive patterns

Strategy: validation

Validate before calling

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

Prevention

Never template a ':' without a guaranteed non-empty name.
Default blank parameter names to a safe constant.
Validate ':' labels have length > 1.

When it happens

Trigger: app.Domain(":.example.com") - a label that is only ":". Also app.Domain("example.com:") is different (a label ending in colon is treated as constant and fails the character check, not this one); this specific panic is the lone-colon label.

Common situations: Templating that emits ":" + name where name is empty; refactoring parameter naming that left a stray colon; user input used as a parameter name that was blank.

Related errors

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/eea7447685490db8.json. Report an issue: GitHub.