diff --git a/src/lib_base/block_header.ml b/src/lib_base/block_header.ml
index 4583aa7d3..d021e83d4 100644
--- a/src/lib_base/block_header.ml
+++ b/src/lib_base/block_header.ml
@@ -14,7 +14,7 @@ type shell_header = {
   timestamp: Time.t ;
   validation_passes: int ; (* uint8 *)
   operations_hash: Operation_list_list_hash.t ;
-  fitness: MBytes.t list ;
+  fitness: Fitness.t ;
   context: Context_hash.t ;
 }
 
diff --git a/src/lib_base/block_header.mli b/src/lib_base/block_header.mli
index 3076307f7..1e8f54b2b 100644
--- a/src/lib_base/block_header.mli
+++ b/src/lib_base/block_header.mli
@@ -14,7 +14,7 @@ type shell_header = {
   timestamp: Time.t ;
   validation_passes: int ; (* uint8 *)
   operations_hash: Operation_list_list_hash.t ;
-  fitness: MBytes.t list ;
+  fitness: Fitness.t ;
   context: Context_hash.t ;
 }