diff --git a/act/model/action.go b/act/model/action.go
index b174e955..50bbc905 100644
--- a/act/model/action.go
+++ b/act/model/action.go
@@ -72,6 +72,22 @@ type ActionRuns struct {
 	Steps      []Step            `yaml:"steps"`
 }
 
+func (actionRuns *ActionRuns) UnmarshalYAML(value *yaml.Node) error {
+	type ActionRunsDefault ActionRuns
+	if err := value.Decode((*ActionRunsDefault)(actionRuns)); err != nil {
+		return err
+	}
+	for i := range actionRuns.Steps {
+		step := &actionRuns.Steps[i]
+		// Set `Number` and  `ID` on each step based upon their position in the steps array:
+		if step.ID == "" {
+			step.ID = fmt.Sprintf("%d", i)
+		}
+		step.Number = i
+	}
+	return nil
+}
+
 // Action describes a metadata file for GitHub actions. The metadata filename must be either action.yml or action.yaml. The data in the metadata file defines the inputs, outputs and main entrypoint for your action.
 type Action struct {
 	Name        string            `yaml:"name"`
diff --git a/act/model/workflow.go b/act/model/workflow.go
index c9acd375..a9a2d32b 100644
--- a/act/model/workflow.go
+++ b/act/model/workflow.go
@@ -594,6 +594,24 @@ func (j *Job) Type() (JobType, error) {
 	return JobTypeDefault, nil
 }
 
+func (j *Job) UnmarshalYAML(value *yaml.Node) error {
+	type JobDefault Job
+	if err := value.Decode((*JobDefault)(j)); err != nil {
+		return err
+	}
+	for i, step := range j.Steps {
+		if step == nil {
+			continue
+		}
+		// Set `Number` and  `ID` on each step based upon their position in the steps array:
+		if step.ID == "" {
+			step.ID = fmt.Sprintf("%d", i)
+		}
+		step.Number = i
+	}
+	return nil
+}
+
 // ContainerSpec is the specification of the container to use for the job
 type ContainerSpec struct {
 	Image       string            `yaml:"image"`
diff --git a/act/runner/action_composite.go b/act/runner/action_composite.go
index fdee793d..45481cd4 100644
--- a/act/runner/action_composite.go
+++ b/act/runner/action_composite.go
@@ -136,17 +136,16 @@ func (rc *RunContext) compositeExecutor(action *model.Action) *compositeSteps {
 
 	sf := &stepFactoryImpl{}
 
-	for i, step := range action.Runs.Steps {
-		if step.ID == "" {
-			step.ID = fmt.Sprintf("%d", i)
+	for i, stepModel := range action.Runs.Steps {
+		if stepModel.Number != i {
+			return &compositeSteps{
+				pre:  func(ctx context.Context) error { return nil },
+				main: common.NewErrorExecutor(fmt.Errorf("internal error: invalid Step: Number expected %v, was actually %v", i, stepModel.Number)),
+				post: func(ctx context.Context) error { return nil },
+			}
 		}
-		step.Number = i
 
-		// create a copy of the step, since this composite action could
-		// run multiple times and we might modify the instance
-		stepcopy := step
-
-		step, err := sf.newStep(&stepcopy, rc)
+		step, err := sf.newStep(&stepModel, rc)
 		if err != nil {
 			return &compositeSteps{
 				main: common.NewErrorExecutor(err),
diff --git a/act/runner/job_executor.go b/act/runner/job_executor.go
index 3cb4f206..ebecf31c 100644
--- a/act/runner/job_executor.go
+++ b/act/runner/job_executor.go
@@ -57,14 +57,10 @@ func newJobExecutor(info jobInfo, sf stepFactory, rc *RunContext) common.Executo
 
 	for i, stepModel := range infoSteps {
 		if stepModel == nil {
-			return func(ctx context.Context) error {
-				return fmt.Errorf("invalid Step %v: missing run or uses key", i)
-			}
+			return common.NewErrorExecutor(fmt.Errorf("invalid Step %v: missing run or uses key", i))
+		} else if stepModel.Number != i {
+			return common.NewErrorExecutor(fmt.Errorf("internal error: invalid Step: Number expected %v, was actually %v", i, stepModel.Number))
 		}
-		if stepModel.ID == "" {
-			stepModel.ID = fmt.Sprintf("%d", i)
-		}
-		stepModel.Number = i
 
 		step, err := sf.newStep(stepModel, rc)
 		if err != nil {
diff --git a/act/runner/job_executor_test.go b/act/runner/job_executor_test.go
index 918ece94..efe35e65 100644
--- a/act/runner/job_executor_test.go
+++ b/act/runner/job_executor_test.go
@@ -209,11 +209,14 @@ func TestJobExecutorNewJobExecutor(t *testing.T) {
 		{
 			name: "stepsWithPreAndPost",
 			steps: []*model.Step{{
-				ID: "1",
+				ID:     "1",
+				Number: 0,
 			}, {
-				ID: "2",
+				ID:     "2",
+				Number: 1,
 			}, {
-				ID: "3",
+				ID:     "3",
+				Number: 2,
 			}},
 			preSteps:  []bool{true, false, true},
 			postSteps: []bool{false, true, true},