diff --git a/src/learner.cc b/src/learner.cc
index 1c0e91deae88..835ea4ad2fe0 100644
--- a/src/learner.cc
+++ b/src/learner.cc
@@ -777,7 +777,8 @@ class LearnerIO : public LearnerConfiguration {
       }
       extra_attr.emplace_back("metrics", os.str());
     }
-
+    std::string header {"binf"};
+    fo->Write(header.data(), 4);
     fo->Write(&mparam, sizeof(LearnerModelParamLegacy));
     fo->Write(tparam_.objective);
     fo->Write(tparam_.booster);