diff --git a/src/testing/testing.go b/src/testing/testing.go
index b64286c005..c7d51a13e3 100644
--- a/src/testing/testing.go
+++ b/src/testing/testing.go
@@ -1187,7 +1187,8 @@ func removeAll(path string) error {
 // restore the environment variable to its original value
 // after the test.
 //
-// This cannot be used in parallel tests.
+// Because Setenv affects the whole process, it cannot be used
+// in parallel tests or tests with parallel ancestors.
 func (c *common) Setenv(key, value string) {
 	c.checkFuzzFn("Setenv")
 	prevValue, ok := os.LookupEnv(key)
@@ -1324,7 +1325,8 @@ func (t *T) Parallel() {
 // restore the environment variable to its original value
 // after the test.
 //
-// This cannot be used in parallel tests.
+// Because Setenv affects the whole process, it cannot be used
+// in parallel tests or tests with parallel ancestors.
 func (t *T) Setenv(key, value string) {
 	// Non-parallel subtests that have parallel ancestors may still
 	// run in parallel with other tests: they are only non-parallel