What causes "Invalid math statement. Expected 1 operands" in dgraph-io/dgraph?

Ensure every unary operator/function has exactly one operand: math(sqrt(a)), math(-a).

Invalid math statement. Expected 1 operands

Error message

Invalid math statement. Expected 1 operands

What it means

A unary math operator (exp, ln, u-, sqrt, floor, ceil, since) was being evaluated but the value stack had no operand to pop. The unary operator is starved of its single operand, indicating a malformed math expression.

Source

Thrown at dql/math.go:112

			return g == 1
		}
		return false
	}

	return false
}

func evalMathStack(opStack, valueStack *mathTreeStack) error {
	topOp, err := opStack.pop()
	if err != nil {
		return errors.Errorf("Invalid Math expression")
	}
	switch {
	case isUnary(topOp.Fn):
		// Since "not" is a unary operator, just pop one value.
		topVal, err := valueStack.pop()
		if err != nil {
			return errors.Errorf("Invalid math statement. Expected 1 operands")
		}
		if opStack.size() > 1 {
			peek := opStack.peek().Fn
			if (peek == "/" || peek == "%") && isZero(topOp.Fn, topVal.Const) {
				return errors.Errorf("Division by zero")
			}
		}
		topOp.Child = []*MathTree{topVal}

	case isTernary(topOp.Fn):
		if valueStack.size() < 3 {
			return errors.Errorf("Invalid Math expression. Expected 3 operands")
		}
		topVal1 := valueStack.popAssert()
		topVal2 := valueStack.popAssert()
		topVal3 := valueStack.popAssert()
		topOp.Child = []*MathTree{topVal3, topVal2, topVal1}

View on GitHub (pinned to 759e242be6)

Solutions

Ensure every unary operator/function has exactly one operand: math(sqrt(a)), math(-a).
Fix dangling operators at the end of the expression like math(a -).
Check unary minus is written directly before a value: math(-a + b).
Validate the query in a REPL/ small query first to isolate the failing expression.

Example fix

// before
u as math(a * )
// after
u as math(a * sqrt(b))

Defensive patterns

Strategy: validation

Validate before calling

// Ensure unary ops/functions have an operand
const expr = 'sqrt(a)';
const unary = ['sqrt','exp','ln','floor','ceil','since'];
for (const f of unary) {
  const re = new RegExp(f + '\\s*\\(\\s*\\)');
  if (re.test(expr)) throw new Error(`${f} is missing its operand`);
}
if (/[+\-*/%]\s*([),]|$)/.test(expr)) throw new Error('Dangling operator without operand');

Try / catch

try {
  return await dgraph.query(query);
} catch (err) {
  if (/Expected 1 operands/.test(String(err))) {
    throw new Error('A unary math operator is missing its operand in: ' + query);
  }
  throw err;
}

Prevention

Write unary minus directly attached to a value (-a, not a -)
Never leave a trailing operator at the end of a math expression
Always parenthesize function arguments: sqrt(a), not sqrt a
Lint generated DQL for dangling operators before submission

When it happens

Trigger: Writing math(a -) or math(sqrt) with no operand following/preceding the unary operator, or an expression where a unary function ends up with an empty sub-expression, e.g. math(a * - ).

Common situations: Hand-edited DQL where an operand was deleted; negation with missing value: math(u- ) or trailing '-'; generated queries with empty variable values interpolated.

Related errors

AI-assisted analysis of dgraph-io/dgraph@759e242be6 (2026-09-01). Data as JSON: /api/errors/46872d334e67fa5a. Report an issue: GitHub.