diff --git a/knn/iterator.cpp b/knn/iterator.cpp
index 5ea7595b..756ae38e 100644
--- a/knn/iterator.cpp
+++ b/knn/iterator.cpp
@@ -50,6 +50,8 @@ class RowidIteratorKNN_c : public Iterator_i
 };
 
 
+const int RowidIteratorKNN_c::DOCS_PER_CHUNK;
+
 static void SortByRowID ( std::vector<DocDist_t> & dData )
 {
 	static constexpr int RADIX_THRESHOLD = 128;