diff --git a/rtengine/colortemp.h b/rtengine/colortemp.h
index d96e6f5ce..145c8ed20 100644
--- a/rtengine/colortemp.h
+++ b/rtengine/colortemp.h
@@ -21,6 +21,7 @@
 
 #include <cmath>
 #include <map>
+#include <string>
 
 namespace rtengine
 {