diff --git a/src/testcurl/https/tls_multi_thread_mode_test.c b/src/testcurl/https/tls_multi_thread_mode_test.c
index 42842cc0..1598ddc9 100644
--- a/src/testcurl/https/tls_multi_thread_mode_test.c
+++ b/src/testcurl/https/tls_multi_thread_mode_test.c
@@ -67,7 +67,7 @@ https_transfer_thread_adapter (void *args)
  * TODO : make client_count a parameter - numver of curl client threads to spawn
  */
 static int
-test_single_client (void *cls, char *cipher_suite,
+test_single_client (void *cls, const char *cipher_suite,
                     int curl_proto_version)
 {
   void *client_thread_ret;
@@ -89,7 +89,7 @@ test_single_client (void *cls, char *cipher_suite,
  * TODO : make client_count a parameter - numver of curl client threads to spawn
  */
 static int
-test_parallel_clients (void *cls, char *cipher_suite,
+test_parallel_clients (void *cls, const char *cipher_suite,
                        int curl_proto_version)
 {
   int i;
diff --git a/src/testcurl/https/tls_test_common.h b/src/testcurl/https/tls_test_common.h
index 4e6abff8..1556839b 100644
--- a/src/testcurl/https/tls_test_common.h
+++ b/src/testcurl/https/tls_test_common.h
@@ -51,7 +51,7 @@
 struct https_test_data
 {
   void *cls;
-  char *cipher_suite;
+  const char *cipher_suite;
   int proto_version;
 };
 
diff --git a/src/testcurl/https/tls_thread_mode_test.c b/src/testcurl/https/tls_thread_mode_test.c
index dc0edcab..6bab4510 100644
--- a/src/testcurl/https/tls_thread_mode_test.c
+++ b/src/testcurl/https/tls_thread_mode_test.c
@@ -68,7 +68,7 @@ https_transfer_thread_adapter (void *args)
  * TODO : make client_count a parameter - numver of curl client threads to spawn
  */
 static int
-test_single_client (void *cls, char *cipher_suite,
+test_single_client (void *cls, const char *cipher_suite,
                     int curl_proto_version)
 {
   void *client_thread_ret;
@@ -89,7 +89,7 @@ test_single_client (void *cls, char *cipher_suite,
  * TODO : make client_count a parameter - numver of curl client threads to spawn
  */
 static int
-test_parallel_clients (void * cls, char *cipher_suite,
+test_parallel_clients (void * cls, const char *cipher_suite,
                        int curl_proto_version)
 {
   int i;