diff --git a/tests/sampler/test_parfor.cpp b/tests/sampler/test_parfor.cpp
index 6535d147c..e46c25aba 100644
--- a/tests/sampler/test_parfor.cpp
+++ b/tests/sampler/test_parfor.cpp
@@ -1,3 +1,4 @@
+#include <string>
 #include <iostream>
 #include <sstream>
 #include <vector>