diff --git a/src/libpcp/src/fetch.c b/src/libpcp/src/fetch.c
index 84deb0c..eb02fa4 100644
--- a/src/libpcp/src/fetch.c
+++ b/src/libpcp/src/fetch.c
@@ -164,7 +164,8 @@ pmFetch(int numpmid, pmID pmidlist[], pmResult **result)
 
 	/* process derived metrics, if any */
 	if (have_dm) {
-	    __dmpostfetch(ctxp, result);
+	    if (n >= 0)
+		__dmpostfetch(ctxp, result);
 	    if (newlist != NULL) {
 		free(newlist);
 	    }