What causes "illegal number syntax: %q" in gohugoio/hugo?

Correct the numeric literal to valid Go constant syntax.

illegal number syntax: %q

Error message

illegal number syntax: %q

What it means

The template numeric literal could not be parsed as int, uint, or float - none of the parse paths succeeded, so newNumber returns illegal number syntax. See node.go:713-715.

Source

Thrown at tpl/internal/go_templates/texttemplate/parse/node.go:714

			// it's a huge number too large to fit in an int. Reject it.
			if !strings.ContainsAny(text, ".eEpP") {
				return nil, fmt.Errorf("integer overflow: %q", text)
			}
			n.IsFloat = true
			n.Float64 = f
			// If a floating-point extraction succeeded, extract the int if needed.
			if !n.IsInt && float64(int64(f)) == f {
				n.IsInt = true
				n.Int64 = int64(f)
			}
			if !n.IsUint && float64(uint64(f)) == f {
				n.IsUint = true
				n.Uint64 = uint64(f)
			}
		}
	}
	if !n.IsInt && !n.IsUint && !n.IsFloat {
		return nil, fmt.Errorf("illegal number syntax: %q", text)
	}
	return n, nil
}

// simplifyComplex pulls out any other types that are represented by the complex number.
// These all require that the imaginary part be zero.
func (n *NumberNode) simplifyComplex() {
	n.IsFloat = imag(n.Complex128) == 0
	if n.IsFloat {
		n.Float64 = real(n.Complex128)
		n.IsInt = float64(int64(n.Float64)) == n.Float64
		if n.IsInt {
			n.Int64 = int64(n.Float64)
		}
		n.IsUint = float64(uint64(n.Float64)) == n.Float64
		if n.IsUint {
			n.Uint64 = uint64(n.Float64)
		}

View on GitHub (pinned to 52c9bd7908)

Solutions

Correct the numeric literal to valid Go constant syntax.
Remove stray characters and fix decimal/thousand separators.
Wrap intended non-numeric text in quotes so it is not parsed as a number.

Example fix

// before
{{ 1,000 }}

// after
{{ 1000 }}

Defensive patterns

Strategy: try-catch

Try / catch

treeSet, err := parse.Parse(name, text, left, right, funcs)
if err != nil {
    // err already formatted as template: name:line: msg
    return err
}

Prevention

Treat the file:line in the formatted message as the authoritative pointer to the real defect.
Fix the underlying specific error described after the line number rather than the wrapper text.

When it happens

Trigger: Malformed literals such as {{ 0xZZ }}, {{ 1..2 }}, {{ 0b2 }}, or locale-formatted numbers with commas like {{ 1,000 }}.

Common situations: Typo in a numeric literal; copy-pasting locale-formatted numbers; stray punctuation inside an action; attempting unsupported base prefixes.

Related errors

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/d012a193d29b5c37. Report an issue: GitHub.