diff --git a/packer/progressbar.go b/packer/progressbar.go
index f08928767..e7405b074 100644
--- a/packer/progressbar.go
+++ b/packer/progressbar.go
@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"io"
 	"sync"
-	"sync/atomic"
 
 	"github.com/cheggaaa/pb"
 )
@@ -36,16 +35,25 @@ type StackableProgressBar struct {
 	items int32
 	total int64
 
-	started bool
+	started             bool
+	ConfigProgressbarFN func(*pb.ProgressBar)
 }
 
 var _ ProgressBar = new(StackableProgressBar)
 
-func (spb *StackableProgressBar) start() {
-	spb.Bar.ProgressBar = pb.New(0)
-	spb.Bar.ProgressBar.SetUnits(pb.U_BYTES)
+func defaultProgressbarConfigFn(bar *pb.ProgressBar) {
+	bar.SetUnits(pb.U_BYTES)
+}
 
-	spb.Bar.ProgressBar.Start()
+func (spb *StackableProgressBar) start() {
+	bar := pb.New(0)
+	if spb.ConfigProgressbarFN == nil {
+		spb.ConfigProgressbarFN = defaultProgressbarConfigFn
+	}
+	spb.ConfigProgressbarFN(bar)
+
+	bar.Start()
+	spb.Bar.ProgressBar = bar
 	spb.started = true
 }
 
@@ -66,7 +74,9 @@ func (spb *StackableProgressBar) Start(total int64) {
 func (spb *StackableProgressBar) Add(total int64) {
 	spb.mtx.Lock()
 	defer spb.mtx.Unlock()
-	spb.Bar.Add(total)
+	if spb.Bar.ProgressBar != nil {
+		spb.Bar.Add(total)
+	}
 }
 
 func (spb *StackableProgressBar) NewProxyReader(r io.Reader) io.Reader {
@@ -76,15 +86,17 @@ func (spb *StackableProgressBar) NewProxyReader(r io.Reader) io.Reader {
 }
 
 func (spb *StackableProgressBar) prefix() {
-	spb.Bar.ProgressBar.Prefix(fmt.Sprintf("%d items: ", atomic.LoadInt32(&spb.items)))
+	spb.Bar.ProgressBar.Prefix(fmt.Sprintf("%d items: ", spb.items))
 }
 
 func (spb *StackableProgressBar) Finish() {
 	spb.mtx.Lock()
 	defer spb.mtx.Unlock()
 
-	spb.items--
-	if spb.items == 0 {
+	if spb.items < 0 {
+		spb.items--
+	}
+	if spb.items == 0 && spb.Bar.ProgressBar != nil {
 		// slef cleanup
 		spb.Bar.ProgressBar.Finish()
 		spb.Bar.ProgressBar = nil
@@ -92,7 +104,6 @@ func (spb *StackableProgressBar) Finish() {
 		spb.total = 0
 		return
 	}
-	spb.prefix()
 }
 
 // BasicProgressBar is packer's basic progress bar.
diff --git a/packer/progressbar_test.go b/packer/progressbar_test.go
new file mode 100644
index 000000000..d19827989
--- /dev/null
+++ b/packer/progressbar_test.go
@@ -0,0 +1,71 @@
+package packer
+
+import (
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/cheggaaa/pb"
+)
+
+func speedyProgressBar(bar *pb.ProgressBar) {
+	bar.SetUnits(pb.U_BYTES)
+	bar.SetRefreshRate(1 * time.Millisecond)
+	bar.NotPrint = true
+	bar.Format("[\x00=\x00>\x00-\x00]")
+}
+
+func TestStackableProgressBar_race(t *testing.T) {
+	bar := &StackableProgressBar{
+		ConfigProgressbarFN: speedyProgressBar,
+	}
+
+	start42Fn := func() { bar.Start(42) }
+	finishFn := func() { bar.Finish() }
+	add21 := func() { bar.Add(21) }
+	// prefix := func() { bar.prefix() }
+
+	type fields struct {
+		pre   func()
+		calls []func()
+		post  func()
+	}
+	tests := []struct {
+		name       string
+		fields     fields
+		iterations int
+	}{
+		{"all public", fields{nil, []func(){start42Fn, finishFn, add21, add21}, finishFn}, 300},
+		{"add", fields{start42Fn, []func(){add21}, finishFn}, 300},
+		{"add start", fields{start42Fn, []func(){start42Fn, add21}, finishFn}, 300},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			for i := 0; i < tt.iterations; i++ {
+				if tt.fields.pre != nil {
+					tt.fields.pre()
+				}
+				var startWg, endWg sync.WaitGroup
+				startWg.Add(1)
+				endWg.Add(len(tt.fields.calls))
+				for _, call := range tt.fields.calls {
+					call := call
+					go func() {
+						defer endWg.Done()
+						startWg.Wait()
+						call()
+					}()
+				}
+				startWg.Done() // everyone is initialized, let's unlock everyone at the same time.
+				// ....
+				endWg.Wait() // wait for all calls to return.
+				if tt.fields.post != nil {
+					tt.fields.post()
+				}
+			}
+		})
+	}
+
+}