diff --git a/dist/index.js b/dist/index.js
index a4a3b09..244c4d0 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -21,10 +21,16 @@ module.exports = class Runner {
    *
    */
   start() {return __async(function*(){
-    const startTask = this.getTask(this.workflow.startTaskId)
+    const startTaskId = this.workflow.startTaskId
+
+    if (!startTaskId) {
+      throw new Error('`startTaskId` must be specified')
+    }
+
+    const startTask = this.getTask(startTaskId)
 
     if (!startTask) {
-      throw new Error('Invalid workflow - missing start task')
+      throw new Error('Invalid workflow - start task not found')
     }
 
     yield this.runTask(startTask)
@@ -106,10 +112,12 @@ module.exports = class Runner {
   }.call(this))}
 
   getTask(id, source) {
-    source = source || this.workflow.tasks
+    source = source || this.workflow.tasks || {}
     const task = source[id]
 
-    task._id = id
+    if (task) {
+      task._id = id
+    }
 
     return task
   }
diff --git a/lib/index.js b/lib/index.js
index 1bfae12..622c282 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -21,10 +21,16 @@ module.exports = class Runner {
    *
    */
   async start() {
-    const startTask = this.getTask(this.workflow.startTaskId)
+    const startTaskId = this.workflow.startTaskId
+
+    if (!startTaskId) {
+      throw new Error('`startTaskId` must be specified')
+    }
+
+    const startTask = this.getTask(startTaskId)
 
     if (!startTask) {
-      throw new Error('Invalid workflow - missing start task')
+      throw new Error('Invalid workflow - start task not found')
     }
 
     await this.runTask(startTask)
@@ -106,10 +112,12 @@ module.exports = class Runner {
   }
 
   getTask(id, source) {
-    source = source || this.workflow.tasks
+    source = source || this.workflow.tasks || {}
     const task = source[id]
 
-    task._id = id
+    if (task) {
+      task._id = id
+    }
 
     return task
   }
diff --git a/test/index.js b/test/index.js
index 58e454d..1b9a073 100644
--- a/test/index.js
+++ b/test/index.js
@@ -10,6 +10,20 @@ test('requires workflow', t => {
   }, 'Invalid workflow specified', 'Throws on invalid workflow')
 })
 
+test('workflow requires startTaskId', async t => {
+  const runner = new Runner({})
+
+  t.throws(runner.start(), '`startTaskId` must be specified',
+    'Throws on invalid')
+})
+
+test('workflow requires startTask', async t => {
+  const runner = new Runner({startTaskId: 'a'})
+
+  t.throws(runner.start(), 'Invalid workflow - start task not found',
+    'Throws on invalid')
+})
+
 test('All pass', async t => {
   const workflow = require('./fixtures/all-pass')