diff --git a/lib/agent.js b/lib/agent.js
index 8bd354e..400bd8b 100644
--- a/lib/agent.js
+++ b/lib/agent.js
@@ -52,6 +52,9 @@ class Agent extends OriginalAgent {
       delete options.freeSocketKeepAliveTimeout;
     }
 
+    // Normalize before deriving the default active socket timeout.
+    options.freeSocketTimeout = ms(options.freeSocketTimeout);
+
     // Sets the socket to timeout after timeout milliseconds of inactivity on the socket.
     // By default is double free socket timeout.
     if (options.timeout === undefined) {
@@ -61,7 +64,6 @@ class Agent extends OriginalAgent {
 
     // support humanize format
     options.timeout = ms(options.timeout);
-    options.freeSocketTimeout = ms(options.freeSocketTimeout);
     options.socketActiveTTL = options.socketActiveTTL ? ms(options.socketActiveTTL) : 0;
 
     super(options);
diff --git a/test/default_timeout.test.js b/test/default_timeout.test.js
new file mode 100644
index 0000000..6ad0f34
--- /dev/null
+++ b/test/default_timeout.test.js
@@ -0,0 +1,94 @@
+'use strict';
+
+const assert = require('assert');
+const http = require('http');
+const https = require('https');
+const fs = require('fs');
+const { HttpAgent, HttpsAgent } = require('..');
+
+[
+  { Agent: HttpAgent, client: http },
+  { Agent: HttpsAgent, client: https },
+].forEach(({ Agent, client }) => {
+  describe(`test/default_timeout.test.js ${Agent.name}`, () => {
+    [ 'freeSocketTimeout', 'keepAliveTimeout', 'freeSocketKeepAliveTimeout' ].forEach(option => {
+      [
+        { value: '10s', freeSocketTimeout: 10000, timeout: 20000 },
+        { value: '100ms', freeSocketTimeout: 100, timeout: 8000 },
+      ].forEach(({ value, freeSocketTimeout, timeout }) => {
+        it(`should derive the default timeout from ${option}: ${value}`, () => {
+          const agent = new Agent({ [option]: value });
+          assert.strictEqual(agent.options.freeSocketTimeout, freeSocketTimeout);
+          assert.strictEqual(agent.options.timeout, timeout);
+        });
+      });
+    });
+
+    [
+      { value: 10000, freeSocketTimeout: 10000, timeout: 20000 },
+      { value: '10000', freeSocketTimeout: 10000, timeout: 20000 },
+      { value: 0, freeSocketTimeout: 0, timeout: 8000 },
+      { value: undefined, freeSocketTimeout: 4000, timeout: 8000 },
+    ].forEach(({ value, freeSocketTimeout, timeout }) => {
+      it(`should preserve the default timeout for freeSocketTimeout: ${value}`, () => {
+        const agent = new Agent({ freeSocketTimeout: value });
+        assert.strictEqual(agent.options.freeSocketTimeout, freeSocketTimeout);
+        assert.strictEqual(agent.options.timeout, timeout);
+      });
+    });
+
+    [ 500, '20s', 0 ].forEach(timeout => {
+      it(`should preserve explicit timeout: ${timeout}`, () => {
+        const agent = new Agent({ freeSocketTimeout: '10s', timeout });
+        assert.strictEqual(agent.options.freeSocketTimeout, 10000);
+        assert.strictEqual(agent.options.timeout, timeout === '20s' ? 20000 : timeout);
+      });
+    });
+
+    it('should derive the default timeout after applying legacy option precedence', () => {
+      const agent = new Agent({
+        freeSocketTimeout: '5s',
+        keepAliveTimeout: '6s',
+        freeSocketKeepAliveTimeout: '10s',
+      });
+      assert.strictEqual(agent.options.freeSocketTimeout, 10000);
+      assert.strictEqual(agent.options.timeout, 20000);
+    });
+
+    describe('working socket', () => {
+      let server;
+      let agent;
+
+      before(done => {
+        const onRequest = req => {
+          // Leave the request active until the agent's inactivity timeout expires.
+          req.resume();
+        };
+        server = client === https ? https.createServer({
+          key: fs.readFileSync(__dirname + '/fixtures/agenttest-key.pem'),
+          cert: fs.readFileSync(__dirname + '/fixtures/agenttest-cert.pem'),
+        }, onRequest) : http.createServer(onRequest);
+        server.listen(0, done);
+      });
+
+      afterEach(() => agent.destroy());
+      after(done => server.close(done));
+
+      it('should time out an active request with a humanized free socket timeout', function(done) {
+        this.timeout(12000);
+        agent = new Agent({ freeSocketTimeout: '100ms' });
+        client.get({
+          agent,
+          port: server.address().port,
+          rejectUnauthorized: false,
+        }, () => {
+          done(new Error('should not receive a response'));
+        }).on('error', err => {
+          assert.strictEqual(err.code, 'ERR_SOCKET_TIMEOUT');
+          assert.strictEqual(err.timeout, 8000);
+          done();
+        });
+      });
+    });
+  });
+});