diff --git a/dashboard/app/build/build.go b/dashboard/app/build/build.go
index 90ca344bd8..9e965edba0 100644
--- a/dashboard/app/build/build.go
+++ b/dashboard/app/build/build.go
@@ -94,6 +94,7 @@ type Commit struct {
 	Time              time.Time
 	NeedsBenchmarking bool
 	TryPatch          bool
+	Branch            string
 
 	// ResultData is the Data string of each build Result for this Commit.
 	// For non-Go commits, only the Results for the current Go tip, weekly,
diff --git a/dashboard/app/build/handler.go b/dashboard/app/build/handler.go
index ee75ba9b77..931ffc12f1 100644
--- a/dashboard/app/build/handler.go
+++ b/dashboard/app/build/handler.go
@@ -183,6 +183,11 @@ func addCommit(c appengine.Context, com *Commit) error {
 		if n == 0 {
 			return errors.New("parent commit not found")
 		}
+	} else if com.Num != 1 {
+		// This is the first commit; fail if it is not number 1.
+		// (This will happen if we try to upload a new/different repo
+		// where there is already commit data. A bad thing to do.)
+		return errors.New("this package already has a first commit; aborting")
 	}
 	// update the tip Tag if this is the Go repo and this isn't on a release branch
 	if p.Path == "" && !strings.HasPrefix(com.Desc, "[") && !isUpdate {