diff --git a/libclingo-lpx/src/parsing.cc b/libclingo-lpx/src/parsing.cc
index 6fc49ab..0068df4 100644
--- a/libclingo-lpx/src/parsing.cc
+++ b/libclingo-lpx/src/parsing.cc
@@ -6,6 +6,7 @@
 #include <algorithm>
 #include <iterator>
 #include <map>
+#include <optional>
 #include <regex>
 #include <stdexcept>
 #include <unordered_map>