diff --git a/src/EquationTags.hh b/src/EquationTags.hh
index 66ea2f1b566a3435457c351f98a5aa61f5f49f01..2989fd7fe031aa0a395eb0a0b3d9587649152d30 100644
--- a/src/EquationTags.hh
+++ b/src/EquationTags.hh
@@ -24,6 +24,7 @@ using namespace std;
 
 #include <map>
 #include <set>
+#include <string>
 
 class EquationTags
 {