diff --git a/ssl/test/runner/key_agreement.go b/ssl/test/runner/key_agreement.go
index d1ec91f..5b6f240 100644
--- a/ssl/test/runner/key_agreement.go
+++ b/ssl/test/runner/key_agreement.go
@@ -165,7 +165,9 @@
 		return nil, nil, err
 	}
 	if bad == RSABadValueCorrupt {
-		encrypted[0] ^= 1
+		encrypted[len(encrypted)-1] ^= 1
+		// Clear the high byte to ensure |encrypted| is still below the RSA modulus.
+		encrypted[0] = 0
 	}
 	ckx := new(clientKeyExchangeMsg)
 	if clientHello.vers != VersionSSL30 {