diff --git a/src/backend/executor/execExpr.c b/src/backend/executor/execExpr.c
index e4e05753ee..ea35103dd6 100644
--- a/src/backend/executor/execExpr.c
+++ b/src/backend/executor/execExpr.c
@@ -2394,6 +2394,7 @@ ExecComputeSlotInfo(ExprState *state, ExprEvalStep *op)
 		{
 			isfixed = true;
 			tts_ops = parent->innerops;
+			desc = ExecGetResultType(is);
 		}
 		else if (is)
 		{
@@ -2413,6 +2414,7 @@ ExecComputeSlotInfo(ExprState *state, ExprEvalStep *op)
 		{
 			isfixed = true;
 			tts_ops = parent->outerops;
+			desc = ExecGetResultType(os);
 		}
 		else if (os)
 		{