diff --git a/src/libslic3r/ObjectID.cpp b/src/libslic3r/ObjectID.cpp
index 7188f05fd..7177c47fe 100644
--- a/src/libslic3r/ObjectID.cpp
+++ b/src/libslic3r/ObjectID.cpp
@@ -17,7 +17,7 @@ ObjectID wipe_tower_instance_id()
     return mine.id();
 }
 
-size_t ObjectWithTimestamp::s_last_timestamp = 1;
+ObjectWithTimestamp::Timestamp ObjectWithTimestamp::s_last_timestamp = 1;
 
 } // namespace Slic3r