diff --git a/internal/ceres/evaluator.cc b/internal/ceres/evaluator.cc
index c94c62c..4675c4a 100644
--- a/internal/ceres/evaluator.cc
+++ b/internal/ceres/evaluator.cc
@@ -50,7 +50,7 @@
 
 Evaluator* Evaluator::Create(const Evaluator::Options& options,
                              Program* program,
-                             string* error) {
+                             std::string* error) {
   switch (options.linear_solver_type) {
     case DENSE_QR:
     case DENSE_NORMAL_CHOLESKY: