diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index e200dcf472..d345522866 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -1704,10 +1704,10 @@ cost_mergejoin(MergePath *path, PlannerInfo *root, SpecialJoinInfo *sjinfo)
 				innerendsel;
 	Path		sort_path;		/* dummy for result of cost_sort */
 
-	/* Protect some assumptions below that rowcounts aren't zero */
-	if (outer_path_rows <= 0)
+	/* Protect some assumptions below that rowcounts aren't zero or NaN */
+	if (outer_path_rows <= 0 || isnan(outer_path_rows))
 		outer_path_rows = 1;
-	if (inner_path_rows <= 0)
+	if (inner_path_rows <= 0 || isnan(inner_path_rows))
 		inner_path_rows = 1;
 
 	if (!enable_mergejoin)