diff --git a/src/backend/catalog/pg_aggregate.c b/src/backend/catalog/pg_aggregate.c
index 66b4af93bd..246776093e 100644
--- a/src/backend/catalog/pg_aggregate.c
+++ b/src/backend/catalog/pg_aggregate.c
@@ -410,16 +410,17 @@ AggregateCreate(const char *aggName,
 		Oid			combineType;
 
 		/*
-		 * Combine function must have 2 argument, each of which is the trans
-		 * type
+		 * Combine function must have 2 arguments, each of which is the trans
+		 * type.  VARIADIC doesn't affect it.
 		 */
 		fnArgs[0] = aggTransType;
 		fnArgs[1] = aggTransType;
 
-		combinefn = lookup_agg_function(aggcombinefnName, 2, fnArgs,
-										variadicArgType, &combineType);
+		combinefn = lookup_agg_function(aggcombinefnName, 2,
+										fnArgs, InvalidOid,
+										&combineType);
 
-		/* Ensure the return type matches the aggregates trans type */
+		/* Ensure the return type matches the aggregate's trans type */
 		if (combineType != aggTransType)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATATYPE_MISMATCH),
@@ -429,14 +430,14 @@ AggregateCreate(const char *aggName,
 
 		/*
 		 * A combine function to combine INTERNAL states must accept nulls and
-		 * ensure that the returned state is in the correct memory context.
+		 * ensure that the returned state is in the correct memory context. We
+		 * cannot directly check the latter, but we can check the former.
 		 */
 		if (aggTransType == INTERNALOID && func_strict(combinefn))
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
 					 errmsg("combine function with transition type %s must not be declared STRICT",
 							format_type_be(aggTransType))));
-
 	}
 
 	/*
@@ -444,10 +445,11 @@ AggregateCreate(const char *aggName,
 	 */
 	if (aggserialfnName)
 	{
+		/* signature is always serialize(internal) returns bytea */
 		fnArgs[0] = INTERNALOID;
 
 		serialfn = lookup_agg_function(aggserialfnName, 1,
-									   fnArgs, variadicArgType,
+									   fnArgs, InvalidOid,
 									   &rettype);
 
 		if (rettype != BYTEAOID)
@@ -463,11 +465,12 @@ AggregateCreate(const char *aggName,
 	 */
 	if (aggdeserialfnName)
 	{
+		/* signature is always deserialize(bytea, internal) returns internal */
 		fnArgs[0] = BYTEAOID;
 		fnArgs[1] = INTERNALOID;	/* dummy argument for type safety */
 
 		deserialfn = lookup_agg_function(aggdeserialfnName, 2,
-										 fnArgs, variadicArgType,
+										 fnArgs, InvalidOid,
 										 &rettype);
 
 		if (rettype != INTERNALOID)
@@ -770,7 +773,11 @@ AggregateCreate(const char *aggName,
 
 /*
  * lookup_agg_function
- * common code for finding transfn, invtransfn, finalfn, and combinefn
+ * common code for finding aggregate support functions
+ *
+ * fnName: possibly-schema-qualified function name
+ * nargs, input_types: expected function argument types
+ * variadicArgType: type of variadic argument if any, else InvalidOid
  *
  * Returns OID of function, and stores its return type into *rettype
  *
diff --git a/src/backend/executor/nodeAgg.c b/src/backend/executor/nodeAgg.c
index 7624a3ac6e..a27c292d4c 100644
--- a/src/backend/executor/nodeAgg.c
+++ b/src/backend/executor/nodeAgg.c
@@ -2940,8 +2940,8 @@ build_pertrans_for_aggref(AggStatePerTrans pertrans,
 		if (pertrans->transfn.fn_strict && aggtranstype == INTERNALOID)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
-					 errmsg("combine function for aggregate %u must be declared as STRICT",
-							aggref->aggfnoid)));
+					 errmsg("combine function with transition type %s must not be declared STRICT",
+							format_type_be(aggtranstype))));
 	}
 	else
 	{