diff --git a/source/detail/xlsx_crypto.cpp b/source/detail/xlsx_crypto.cpp
index 77039331..29f9bed9 100644
--- a/source/detail/xlsx_crypto.cpp
+++ b/source/detail/xlsx_crypto.cpp
@@ -98,7 +98,7 @@ namespace detail {
 
 struct crypto_helper
 {
-    static const std::size_t segment_length = 4096;
+    static const std::size_t segment_length;
 
     enum class cipher_algorithm
     {
@@ -715,6 +715,8 @@ struct crypto_helper
     }
 };
 
+const std::size_t crypto_helper::segment_length = 4096;
+
 void xlsx_consumer::read(std::istream &source, const std::string &password)
 {
 	std::vector<std::uint8_t> data((std::istreambuf_iterator<char>(source)),