diff --git a/dlib/image_processing/object_detector.h b/dlib/image_processing/object_detector.h
index 5272fe1f2..23993da74 100644
--- a/dlib/image_processing/object_detector.h
+++ b/dlib/image_processing/object_detector.h
@@ -7,6 +7,7 @@
 #include "../matrix.h"
 #include "../geometry.h"
 #include <vector>
+#include "box_overlap_testing.h"
 
 namespace dlib
 {
@@ -15,7 +16,7 @@ namespace dlib
 
     template <
         typename image_scanner_type,
-        typename overlap_tester_type
+        typename overlap_tester_type = test_box_overlap
         >
     class object_detector
     {
diff --git a/dlib/image_processing/object_detector_abstract.h b/dlib/image_processing/object_detector_abstract.h
index 3739b32b5..3ed3b506d 100644
--- a/dlib/image_processing/object_detector_abstract.h
+++ b/dlib/image_processing/object_detector_abstract.h
@@ -6,6 +6,7 @@
 #include "../matrix.h"
 #include "../geometry.h"
 #include <vector>
+#include "box_overlap_testing_abstract.h"
 
 namespace dlib
 {
@@ -14,7 +15,7 @@ namespace dlib
 
     template <
         typename image_scanner_type,
-        typename overlap_tester_type
+        typename overlap_tester_type = test_box_overlap
         >
     class object_detector
     {