diff --git a/Foundation/src/RegularExpression.cpp b/Foundation/src/RegularExpression.cpp
index ffde2792a..0ab62b654 100644
--- a/Foundation/src/RegularExpression.cpp
+++ b/Foundation/src/RegularExpression.cpp
@@ -26,7 +26,7 @@
 namespace Poco {
 
 
-const int RegularExpression::OVEC_SIZE = 64;
+const int RegularExpression::OVEC_SIZE = 63; // must be multiple of 3
 
 
 RegularExpression::RegularExpression(const std::string& pattern, int options, bool study): _pcre(0), _extra(0)