diff --git a/common.go b/common.go
index dc4adae..9ea149a 100644
--- a/common.go
+++ b/common.go
@@ -597,10 +597,13 @@ type Config struct {
 	LimitFallbackUpload   LimitFallback
 	LimitFallbackDownload LimitFallback
 
-	// Rand provides the source of entropy for nonces and RSA blinding.
+	// Rand provides the source of entropy for the connection.
 	// If Rand is nil, TLS uses the cryptographic random reader in package
-	// crypto/rand.
-	// The Reader must be safe for use by multiple goroutines.
+	// crypto/rand. The Reader must be safe for use by multiple goroutines.
+	//
+	// Deprecated: this should be left nil in production. Not all TLS
+	// configurations are guaranteed to use Rand. Test code can use
+	// [testing/cryptotest.SetGlobalRandom] instead.
 	Rand io.Reader
 
 	// Time returns the current time as the number of seconds since the epoch.