diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index ef2a6bc6e9d..b852b96839d 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -1514,6 +1514,7 @@ ExecForPortionOfLeftovers(ModifyTableContext *context,
 	rsi.expectedDesc = NULL;
 	rsi.allowedModes = (int) (SFRM_ValuePerCall);
 	rsi.returnMode = SFRM_ValuePerCall;
+	/* isDone is filled below */
 	rsi.setResult = NULL;
 	rsi.setDesc = NULL;
 
@@ -1537,14 +1538,22 @@ ExecForPortionOfLeftovers(ModifyTableContext *context,
 	 */
 	while (true)
 	{
-		Datum		leftover = FunctionCallInvoke(fcinfo);
+		Datum		leftover;
+
+		/* Call the function one time */
+		fcinfo->isnull = false;
+		rsi.isDone = ExprSingleResult;
+		leftover = FunctionCallInvoke(fcinfo);
+
+		if (rsi.returnMode != SFRM_ValuePerCall)
+			elog(ERROR, "without_portion function violated function call protocol");
 
 		/* Are we done? */
 		if (rsi.isDone == ExprEndResult)
 			break;
 
 		if (fcinfo->isnull)
-			elog(ERROR, "Got a null from without_portion function");
+			elog(ERROR, "got a null from without_portion function");
 
 		/*
 		 * Does the new Datum violate domain checks? Row-level CHECK