diff --git a/internal/ceres/linear_solver.h b/internal/ceres/linear_solver.h
index bdf7b56..67bebe0 100644
--- a/internal/ceres/linear_solver.h
+++ b/internal/ceres/linear_solver.h
@@ -36,6 +36,7 @@
 
 #include <cstddef>
 #include <map>
+#include <string>
 #include <vector>
 #include "ceres/block_sparse_matrix.h"
 #include "ceres/casts.h"