diff --git a/btparser/lexer.h b/btparser/lexer.h
index 975622c..e3e7960 100644
--- a/btparser/lexer.h
+++ b/btparser/lexer.h
@@ -4,6 +4,7 @@
 #include <vector>
 #include <unordered_map>
 #include <functional>
+#include <string>
 
 class Lexer
 {