Index: src/Concurrency/KeywordsNew.cpp
===================================================================
--- src/Concurrency/KeywordsNew.cpp	(revision 0d4f9548207455c4183f179f0e20757dbedfce9b)
+++ src/Concurrency/KeywordsNew.cpp	(revision 41d3c8d53e47a61467628191045f040de3226cb8)
@@ -1004,4 +1004,7 @@
 
 const ast::Stmt * MutexKeyword::postvisit( const ast::MutexStmt * stmt ) {
+	if ( !lock_guard_decl ) {
+		SemanticError( stmt->location, "mutex stmt requires a header, add #include <mutex_stmt.hfa>\n" );
+	}
 	ast::CompoundStmt * body =
 			new ast::CompoundStmt( stmt->location, { stmt->stmt } );