diff --git a/src/include/taler_testing_lib.h b/src/include/taler_testing_lib.h
index 14e09c911..c90991ac4 100644
--- a/src/include/taler_testing_lib.h
+++ b/src/include/taler_testing_lib.h
@@ -2370,7 +2370,8 @@ TALER_TESTING_get_trait_merchant_pub (const struct TALER_TESTING_Command *cmd,
  */
 struct TALER_TESTING_Trait
 TALER_TESTING_make_trait_merchant_pub (unsigned int index,
-                                       struct TALER_MerchantPublicKeyP *pub);
+                                       const struct
+                                       TALER_MerchantPublicKeyP *pub);
 
 
 /**
diff --git a/src/lib/testing_api_trait_merchant_key.c b/src/lib/testing_api_trait_merchant_key.c
index 4dc30c6a2..1657560d3 100644
--- a/src/lib/testing_api_trait_merchant_key.c
+++ b/src/lib/testing_api_trait_merchant_key.c
@@ -112,7 +112,8 @@ TALER_TESTING_get_trait_merchant_pub
  */
 struct TALER_TESTING_Trait
 TALER_TESTING_make_trait_merchant_pub (unsigned int index,
-                                       struct TALER_MerchantPublicKeyP *pub)
+                                       const struct
+                                       TALER_MerchantPublicKeyP *pub)
 {
   struct TALER_TESTING_Trait ret = {
     .index = index,