diff --git a/.gitignore b/.gitignore
index ae40550..210bf24 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,4 @@ composer.lock
 phpunit.xml
 vendor
 .php_cs.cache
+coverage
diff --git a/lib/Config.php b/lib/Config.php
index 94157de..9db9d25 100644
--- a/lib/Config.php
+++ b/lib/Config.php
@@ -31,8 +31,8 @@ class Config {
         $this->attempts = $attempts;
     }
 
-    private function validateNameserver(string $nameserver) {
-        if (!$nameserver) {
+    private function validateNameserver($nameserver) {
+        if (!$nameserver || !\is_string($nameserver)) {
             throw new ConfigException("Invalid nameserver: {$nameserver}");
         }
 
diff --git a/test/ConfigTest.php b/test/ConfigTest.php
new file mode 100644
index 0000000..c58bda2
--- /dev/null
+++ b/test/ConfigTest.php
@@ -0,0 +1,70 @@
+<?php
+
+namespace Amp\Dns\Test;
+
+use Amp\Dns\Config;
+use Amp\Dns\ConfigException;
+use Amp\PHPUnit\TestCase;
+
+class ConfigTest extends TestCase {
+    /**
+     * @param string[] $nameservers Valid server array.
+     *
+     * @dataProvider provideValidServers
+     */
+    public function testAcceptsValidServers(array $nameservers) {
+        $this->assertInstanceOf(Config::class, new Config($nameservers));
+    }
+
+    public function provideValidServers() {
+        return [
+            [["127.1.1.1"]],
+            [["127.1.1.1:1"]],
+            [["[::1]:52"]],
+            [["[::1]"]],
+        ];
+    }
+
+    /**
+     * @param string[] $nameservers Invalid server array.
+     *
+     * @dataProvider provideInvalidServers
+     */
+    public function testRejectsInvalidServers(array $nameservers) {
+        $this->expectException(ConfigException::class);
+        new Config($nameservers);
+    }
+
+    public function provideInvalidServers() {
+        return [
+            [[]],
+            [[42]],
+            [[null]],
+            [[true]],
+            [["foobar"]],
+            [["foobar.com"]],
+            [["127.1.1"]],
+            [["127.1.1.1.1"]],
+            [["126.0.0.5", "foobar"]],
+            [["42"]],
+            [["::1"]],
+            [["::1:53"]],
+            [["[::1]:"]],
+            [["[::1]:76235"]],
+            [["[::1]:0"]],
+            [["[::1]:-1"]],
+            [["[::1:51"]],
+            [["[::1]:abc"]],
+        ];
+    }
+
+    public function testInvalidTimeout() {
+        $this->expectException(ConfigException::class);
+        new Config(["127.0.0.1"], [], -1);
+    }
+
+    public function testInvalidAttempts() {
+        $this->expectException(ConfigException::class);
+        new Config(["127.0.0.1"], [], 500, 0);
+    }
+}