diff --git a/lib/floe/workflow.rb b/lib/floe/workflow.rb
index 5f288f31..9fa6c791 100644
--- a/lib/floe/workflow.rb
+++ b/lib/floe/workflow.rb
@@ -71,8 +71,8 @@ def run_nonblock
 
     def step_nonblock
       return Errno::EPERM if end?
+      return 0 unless step_next
 
-      step_next
       current_state.run_nonblock!
     end
 
@@ -103,7 +103,21 @@ def current_state
     private
 
     def step_next
-      context.state = {"Name" => context.next_state, "Input" => context.output} if context.next_state
+      desired_state = context.next_state || context.state_name
+      if !@states_by_name.key?(desired_state)
+        context.state["Error"] = "States.Runtime"
+        context.state["Cause"] = "State \"#{desired_state}\" does not exist"
+        context.output = {"Error" => context.state["Error"], "Cause" => context.state["Cause"]}
+        context.execution["StartTime"] ||= Time.now.utc
+        context.execution["EndTime"] = Time.now.utc.iso8601
+        return false
+      end
+
+      if context.next_state
+        context.state = {"Name" => context.next_state, "Input" => context.output}
+      end
+
+      true
     end
   end
 end
diff --git a/spec/workflow_spec.rb b/spec/workflow_spec.rb
index 8eb854c7..2f6fa7d2 100644
--- a/spec/workflow_spec.rb
+++ b/spec/workflow_spec.rb
@@ -93,6 +93,26 @@
       expect(ctx.running?).to eq(true)
       expect(ctx.ended?).to   eq(false)
     end
+
+    it "detects invalid StartAt" do
+      workflow = Floe::Workflow.new({"StartAt" => "Bogus", "States" => {"FirstState" => {"Type" => "Succeed"}}}, ctx, {})
+
+      workflow.run_nonblock
+
+      expect(ctx.running?).to eq(false)
+      expect(ctx.ended?).to   eq(true)
+      expect(ctx.status).to eq("failure")
+    end
+
+    it "detects invalid Next" do
+      workflow = Floe::Workflow.new({"StartAt" => "FirstState", "States" => {"FirstState" => {"Type" => "Pass", "Next" => "Bogus"}}}, ctx, {})
+
+      workflow.run_nonblock
+
+      expect(ctx.running?).to eq(false)
+      expect(ctx.ended?).to   eq(true)
+      expect(ctx.status).to eq("failure")
+    end
   end
 
   describe "#step" do