diff --git a/examples/pascal_voc_2012.h b/examples/pascal_voc_2012.h
index ad348c6df..745674f63 100644
--- a/examples/pascal_voc_2012.h
+++ b/examples/pascal_voc_2012.h
@@ -7,6 +7,7 @@
 #define PASCAL_VOC_2012_H_
 
 #include <dlib/pixel.h>
+#include <dlib/dnn.h>
 
 // ----------------------------------------------------------------------------------------