diff --git a/src/backend/executor/execSRF.c b/src/backend/executor/execSRF.c
index 8aec3b549b..545b6c19da 100644
--- a/src/backend/executor/execSRF.c
+++ b/src/backend/executor/execSRF.c
@@ -353,11 +353,21 @@ ExecMakeTableFunctionResult(SetExprState *setexpr,
 			 */
 			if (rsinfo.isDone != ExprMultipleResult)
 				break;
+
+			/*
+			 * Check that set-returning functions were properly declared.
+			 * (Note: for historical reasons, we don't complain if a non-SRF
+			 * returns ExprEndResult; that's treated as returning NULL.)
+			 */
+			if (!returnsSet)
+				ereport(ERROR,
+						(errcode(ERRCODE_E_R_I_E_SRF_PROTOCOL_VIOLATED),
+						 errmsg("table-function protocol for value-per-call mode was not followed")));
 		}
 		else if (rsinfo.returnMode == SFRM_Materialize)
 		{
 			/* check we're on the same page as the function author */
-			if (!first_time || rsinfo.isDone != ExprSingleResult)
+			if (!first_time || rsinfo.isDone != ExprSingleResult || !returnsSet)
 				ereport(ERROR,
 						(errcode(ERRCODE_E_R_I_E_SRF_PROTOCOL_VIOLATED),
 						 errmsg("table-function protocol for materialize mode was not followed")));