diff --git a/lib/precondition.js b/lib/precondition.js
index 1781449..1274dbb 100644
--- a/lib/precondition.js
+++ b/lib/precondition.js
@@ -1,5 +1,6 @@
 'use strict';
 
+var $isFinite = isFinite;
 var MAX_ALLOC = Math.pow(2, 30) - 1; // default in iojs
 
 module.exports = function (iterations, keylen) {
@@ -7,7 +8,7 @@ module.exports = function (iterations, keylen) {
 		throw new TypeError('Iterations not a number');
 	}
 
-	if (iterations < 0) {
+	if (iterations < 0 || !$isFinite(iterations)) {
 		throw new TypeError('Bad iterations');
 	}
 
diff --git a/test/iteration-guard.js b/test/iteration-guard.js
new file mode 100644
index 0000000..b25b3c8
--- /dev/null
+++ b/test/iteration-guard.js
@@ -0,0 +1,158 @@
+'use strict';
+
+var test = require('tape');
+var pbkdf2 = require('..');
+var precondition = require('../lib/precondition');
+var nodeCrypto = require('crypto');
+
+var password = Buffer.from('password');
+var salt = Buffer.from('salt');
+var keylen = 32;
+var digest = 'sha256';
+
+var hasBigInt = typeof BigInt === 'function';
+var makeBigInt = function (x) { return hasBigInt ? BigInt(x) : undefined; };
+
+var RE_NOT_NUMBER = /(iterations?|rounds?)\s*(not a number|must be a number|invalid)/i;
+var RE_BAD_ITER = /(bad iterations?|iterations?\s*(must be finite|must be positive|non[-\s]?finite|>=?\s*0))/i;
+
+test('precondition: rejects non-number iterations', function (t) {
+	var badNotNumber = [
+		'100',
+		true,
+		false,
+		null,
+		undefined,
+		[],
+		{},
+		function () {},
+		Object(10),
+		Object(NaN)
+	];
+	var maybeBig = makeBigInt(10);
+	if (typeof maybeBig !== 'undefined') { badNotNumber.push(maybeBig); }
+
+	t.plan(badNotNumber.length);
+
+	badNotNumber.forEach(function (v) {
+		t['throws'](
+			function () { precondition(v, keylen); },
+			RE_NOT_NUMBER,
+			'precondition: non-number (' + String(v) + ') rejected'
+		);
+	});
+});
+
+test('precondition: rejects non-finite and negative iterations', function (t) {
+	var vals = [Infinity, -Infinity, NaN, -1, -0.5];
+	t.plan(vals.length);
+
+	vals.forEach(function (v) {
+		t['throws'](
+			function () { precondition(v, keylen); },
+			RE_BAD_ITER,
+			'precondition: ' + String(v) + ' rejected as bad iterations'
+		);
+	});
+});
+
+test('precondition: accepts very large finite numbers (no KDF run here)', function (t) {
+	t.plan(2);
+	t.doesNotThrow(
+		function () { precondition(Number.MAX_SAFE_INTEGER, keylen); },
+		'MAX_SAFE_INTEGER passes precondition'
+	);
+	t.doesNotThrow(
+		function () { precondition(Number.MAX_VALUE, keylen); },
+		'MAX_VALUE passes precondition'
+	);
+});
+
+test('API: rejects non-finite iterations (no infinite loop possible)', function (t) {
+	t.plan(6);
+
+	t['throws'](
+		function () { pbkdf2.pbkdf2(password, salt, Infinity, keylen, digest, function () {}); },
+		RE_BAD_ITER,
+		'async: Infinity rejected'
+	);
+	t['throws'](
+		function () { pbkdf2.pbkdf2(password, salt, -Infinity, keylen, digest, function () {}); },
+		RE_BAD_ITER,
+		'async: -Infinity rejected'
+	);
+	t['throws'](
+		function () { pbkdf2.pbkdf2(password, salt, NaN, keylen, digest, function () {}); },
+		RE_BAD_ITER,
+		'async: NaN rejected'
+	);
+
+	t['throws'](
+		function () { pbkdf2.pbkdf2Sync(password, salt, Infinity, keylen, digest); },
+		RE_BAD_ITER,
+		'sync: Infinity rejected'
+	);
+	t['throws'](
+		function () { pbkdf2.pbkdf2Sync(password, salt, -Infinity, keylen, digest); },
+		RE_BAD_ITER,
+		'sync: -Infinity rejected'
+	);
+	t['throws'](
+		function () { pbkdf2.pbkdf2Sync(password, salt, NaN, keylen, digest); },
+		RE_BAD_ITER,
+		'sync: NaN rejected'
+	);
+});
+
+test('API: rejects other unwanted iteration values', function (t) {
+	var notNumberCases = [
+		'"100"',
+		undefined,
+		null,
+		true,
+		Object(7)
+	];
+	var badNumberCases = [-1, -0.25];
+
+	if (hasBigInt) { notNumberCases.push(makeBigInt(10)); }
+
+	t.plan((notNumberCases.length + badNumberCases.length) * 2);
+
+	notNumberCases.forEach(function (val) {
+		t['throws'](
+			function () { pbkdf2.pbkdf2(password, salt, val, keylen, digest, function () {}); },
+			RE_NOT_NUMBER,
+			'async: ' + String(val) + ' rejected (not a number)'
+		);
+		t['throws'](
+			function () { pbkdf2.pbkdf2Sync(password, salt, val, keylen, digest); },
+			RE_NOT_NUMBER,
+			'sync: ' + String(val) + ' rejected (not a number)'
+		);
+	});
+
+	badNumberCases.forEach(function (val) {
+		t['throws'](
+			function () { pbkdf2.pbkdf2(password, salt, val, keylen, digest, function () {}); },
+			RE_BAD_ITER,
+			'async: ' + String(val) + ' rejected (bad iterations)'
+		);
+		t['throws'](
+			function () { pbkdf2.pbkdf2Sync(password, salt, val, keylen, digest); },
+			RE_BAD_ITER,
+			'sync: ' + String(val) + ' rejected (bad iterations)'
+		);
+	});
+});
+
+test('sanity: small finite iterations still match Node crypto', function (t) {
+	t.plan(2);
+	var iterations = 2;
+	var expected = nodeCrypto.pbkdf2Sync(password, salt, iterations, keylen, digest);
+	var actual = pbkdf2.pbkdf2Sync(password, salt, iterations, keylen, digest);
+	t.deepEqual(actual, expected, 'sync derived key matches Node for finite iterations');
+
+	pbkdf2.pbkdf2(password, salt, iterations, keylen, digest, function (err) {
+		t.error(err, 'async derived key succeeds for finite iterations');
+	});
+});