diff --git a/src/testing/testing.go b/src/testing/testing.go
index fcf7048f234..1bc8aff00d1 100644
--- a/src/testing/testing.go
+++ b/src/testing/testing.go
@@ -398,7 +398,7 @@ var initRan bool
 // the "go test" command before running test functions, so Init is only needed
 // when calling functions such as Benchmark without using "go test".
 //
-// Init has no effect if it was already called.
+// Init is not safe to call concurrently. It has no effect if it was already called.
 func Init() {
 	if initRan {
 		return