diff -ur liborcus.org/src/liborcus/xml_context_base.cpp liborcus/src/liborcus/xml_context_base.cpp
--- liborcus.org/src/liborcus/xml_context_base.cpp	2017-06-30 14:32:19.044195127 +0200
+++ liborcus/src/liborcus/xml_context_base.cpp	2017-06-30 14:34:27.580268421 +0200
@@ -171,7 +171,7 @@
 
     // Create a generic error message.
     ostringstream os;
-    os << "element '" << ns << ":" << m_tokens.get_token_name(name) << "' expected, but '";
+    os << "element '" << (ns ? ns : "") << ":" << m_tokens.get_token_name(name) << "' expected, but '";
     os << elem.first << ":" << m_tokens.get_token_name(elem.second) << "' encountered.";
     throw xml_structure_error(os.str());
 }