diff --git a/src/util/string-helper.hpp b/src/util/string-helper.hpp
index ee3f5bd..de0c41f 100644
--- a/src/util/string-helper.hpp
+++ b/src/util/string-helper.hpp
@@ -13,7 +13,7 @@
 
 namespace ndn {
 
-const char *WHITESPACE_CHARS = " \n\r\t";
+static const char *WHITESPACE_CHARS = " \n\r\t";
 
 /**
  * Modify str in place to erase whitespace on the left.