diff --git a/hclpack/positions_packed.go b/hclpack/positions_packed.go
index 0622cc5..29c7144 100644
--- a/hclpack/positions_packed.go
+++ b/hclpack/positions_packed.go
@@ -28,6 +28,7 @@ func (pp positionsPacked) MarshalBinary() ([]byte, error) {
 		// for a body to be entirely in one file, this can lead to considerable
 		// savings in that case.
 		delims := ppr.FileIdx - lastFileIdx
+		lastFileIdx = ppr.FileIdx
 		for i := 0; i < delims; i++ {
 			buf = buf.AppendRawByte(';')
 		}
@@ -65,6 +66,7 @@ func (pp *positionsPacked) UnmarshalBinary(data []byte) error {
 
 		var ppr positionPacked
 		var err error
+		ppr.FileIdx = fileIdx
 		ppr.LineDelta, buf, err = buf.ReadInt()
 		if err != nil {
 			return err
diff --git a/hclpack/positions_packed_test.go b/hclpack/positions_packed_test.go
new file mode 100644
index 0000000..fbb2ca4
--- /dev/null
+++ b/hclpack/positions_packed_test.go
@@ -0,0 +1,30 @@
+package hclpack
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestBinaryRoundTrip(t *testing.T) {
+	startPacked := positionsPacked{
+		{FileIdx: 0, LineDelta: 1, ColumnDelta: 2, ByteDelta: 3},
+		{FileIdx: 1, LineDelta: 2, ColumnDelta: 3, ByteDelta: 4},
+		{FileIdx: 2, LineDelta: 3, ColumnDelta: 4, ByteDelta: 5},
+	}
+
+	b, err := startPacked.MarshalBinary()
+	if err != nil {
+		t.Fatalf("Failed to marshal: %s", err)
+	}
+
+	var endPacked positionsPacked
+	err = endPacked.UnmarshalBinary(b)
+	if err != nil {
+		t.Fatalf("Failed to unmarshal: %s", err)
+	}
+
+	if !cmp.Equal(startPacked, endPacked) {
+		t.Errorf("Incorrect result\n%s", cmp.Diff(startPacked, endPacked))
+	}
+}