diff --git a/magick/threshold.c b/magick/threshold.c
index c16616c..211e6a8 100644
--- a/magick/threshold.c
+++ b/magick/threshold.c
@@ -46,6 +46,7 @@
 #include "magick/cache-view.h"
 #include "magick/color.h"
 #include "magick/color-private.h"
+#include "magick/colormap.h"
 #include "magick/colorspace.h"
 #include "magick/configure.h"
 #include "magick/constitute.h"