Make the argument validation stricter

Closes #6

index.js

@@ -38,16 +38,16 @@ const allowedTypes = [
 ];
 
 module.exports = ({length, type, characters}) => {
-	if (!Number.isFinite(length)) {
-		throw new TypeError('Expected a finite number');
+	if (!(length >= 0 && Number.isFinite(length))) {
+		throw new TypeError('Expected a `length` to be a non-negative finite number');
 	}
 
 	if (type !== undefined && characters !== undefined) {
-		throw new TypeError('Expected either type or characters');
+		throw new TypeError('Expected either `type` or `characters`');
 	}
 
 	if (characters !== undefined && typeof characters !== 'string') {
-		throw new TypeError('Expected characters to be string');
+		throw new TypeError('Expected `characters` to be string');
 	}
 
 	if (!allowedTypes.includes(type)) {

package.json

@@ -33,11 +33,11 @@
 		"hex"
 	],
 	"dependencies": {
-		"type-fest": "^0.4.1"
+		"type-fest": "^0.5.2"
 	},
 	"devDependencies": {
-		"ava": "^1.4.1",
-		"tsd": "^0.7.2",
+		"ava": "^2.1.0",
+		"tsd": "^0.7.3",
 		"xo": "^0.24.0"
 	}
 }

test.js

@@ -41,6 +41,10 @@ test('argument errors', t => {
 		cryptoRandomString({length: Infinity});
 	});
 
+	t.throws(() => {
+		cryptoRandomString({length: -1});
+	});
+
 	t.throws(() => {
 		cryptoRandomString({length: 0, type: 'hex', characters: '1234'});
 	});