diff --git a/include/types/sample.h b/include/types/sample.h
index a2d2296..46bc75b 100644
--- a/include/types/sample.h
+++ b/include/types/sample.h
@@ -260,6 +260,7 @@
 			char **err_msg);          /* argument validation function */
 	unsigned int in_type;                     /* expected input sample type */
 	unsigned int out_type;                    /* output sample type */
+	unsigned int private;                     /* private values. only used by maps */
 };
 
 /* sample conversion expression */