diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 656d03b69aa510e5573ac2b6f0b15cb884ab5bd9..0d5cafba9627f0529731f3f68b077a353832b00a 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -3209,6 +3209,14 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows)
 	double		numdistinct;
 	ListCell   *l;
 
+	/*
+	 * We don't ever want to return an estimate of zero groups, as that tends
+	 * to lead to division-by-zero and other unpleasantness.  The input_rows
+	 * estimate is usually already at least 1, but clamp it just in case it
+	 * isn't.
+	 */
+	input_rows = clamp_row_est(input_rows);
+
 	/*
 	 * If no grouping columns, there's exactly one group.  (This can't happen
 	 * for normal cases with GROUP BY or DISTINCT, but it is possible for