diff --git a/build/ci.go b/build/ci.go
index 5b2e1d98f7..fb5b2e6f11 100644
--- a/build/ci.go
+++ b/build/ci.go
@@ -321,7 +321,7 @@ func doTest(cmdline []string) {
 
 	// Test a single package at a time. CI builders are slow
 	// and some tests run into timeouts under load.
-	gotest.Args = append(gotest.Args, "-p", *threads)
+	gotest.Args = append(gotest.Args, "-p", fmt.Sprintf("%d", *threads))
 	if *coverage {
 		gotest.Args = append(gotest.Args, "-covermode=atomic", "-cover")
 	}