diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index c0a0165c84a..7d034d26415 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -8768,7 +8768,11 @@ ATPrepAlterColumnType(List **wqueue,
 				 errmsg("cannot alter system column \"%s\"",
 						colName)));
 
-	/* Don't alter inherited columns */
+	/*
+	 * Don't alter inherited columns.  At outer level, there had better not be
+	 * any inherited definition; when recursing, we assume this was checked at
+	 * the parent level (see below).
+	 */
 	if (attTup->attinhcount > 0 && !recursing)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
@@ -8892,20 +8896,26 @@ ATPrepAlterColumnType(List **wqueue,
 	if (recurse)
 	{
 		Oid			relid = RelationGetRelid(rel);
-		ListCell   *child;
-		List	   *children;
+		List	   *child_oids,
+				   *child_numparents;
+		ListCell   *lo,
+				   *li;
 
-		children = find_all_inheritors(relid, lockmode, NULL);
+		child_oids = find_all_inheritors(relid, lockmode,
+										 &child_numparents);
 
 		/*
 		 * find_all_inheritors does the recursive search of the inheritance
 		 * hierarchy, so all we have to do is process all of the relids in the
 		 * list that it returns.
 		 */
-		foreach(child, children)
+		forboth(lo, child_oids, li, child_numparents)
 		{
-			Oid			childrelid = lfirst_oid(child);
+			Oid			childrelid = lfirst_oid(lo);
+			int			numparents = lfirst_int(li);
 			Relation	childrel;
+			HeapTuple	childtuple;
+			Form_pg_attribute childattTup;
 
 			if (childrelid == relid)
 				continue;
@@ -8914,6 +8924,29 @@ ATPrepAlterColumnType(List **wqueue,
 			childrel = relation_open(childrelid, NoLock);
 			CheckTableNotInUse(childrel, "ALTER TABLE");
 
+			/*
+			 * Verify that the child doesn't have any inherited definitions of
+			 * this column that came from outside this inheritance hierarchy.
+			 * (renameatt makes a similar test, though in a different way
+			 * because of its different recursion mechanism.)
+			 */
+			childtuple = SearchSysCacheAttName(RelationGetRelid(childrel),
+											   colName);
+			if (!HeapTupleIsValid(childtuple))
+				ereport(ERROR,
+						(errcode(ERRCODE_UNDEFINED_COLUMN),
+						 errmsg("column \"%s\" of relation \"%s\" does not exist",
+								colName, RelationGetRelationName(childrel))));
+			childattTup = (Form_pg_attribute) GETSTRUCT(childtuple);
+
+			if (childattTup->attinhcount > numparents)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
+						 errmsg("cannot alter inherited column \"%s\" of relation \"%s\"",
+								colName, RelationGetRelationName(childrel))));
+
+			ReleaseSysCache(childtuple);
+
 			/*
 			 * Remap the attribute numbers.  If no USING expression was
 			 * specified, there is no need for this step.
diff --git a/src/test/regress/expected/inherit.out b/src/test/regress/expected/inherit.out
index 79ea0c5774e..a5654889806 100644
--- a/src/test/regress/expected/inherit.out
+++ b/src/test/regress/expected/inherit.out
@@ -701,6 +701,16 @@ select * from d;
  32 | one | two | three
 (1 row)
 
+-- The above verified that we can change the type of a multiply-inherited
+-- column; but we should reject that if any definition was inherited from
+-- an unrelated parent.
+create temp table parent1(f1 int, f2 int);
+create temp table parent2(f1 int, f3 bigint);
+create temp table childtab(f4 int) inherits(parent1, parent2);
+NOTICE:  merging multiple inherited definitions of column "f1"
+alter table parent1 alter column f1 type bigint;  -- fail, conflict w/parent2
+ERROR:  cannot alter inherited column "f1" of relation "childtab"
+alter table parent1 alter column f2 type bigint;  -- ok
 -- check that oid column is handled properly during alter table inherit
 create table oid_parent (a int) with oids;
 create table oid_child () inherits (oid_parent);
diff --git a/src/test/regress/sql/inherit.sql b/src/test/regress/sql/inherit.sql
index 101a0394f87..485442fef9d 100644
--- a/src/test/regress/sql/inherit.sql
+++ b/src/test/regress/sql/inherit.sql
@@ -191,6 +191,15 @@ insert into d values('test','one','two','three');
 alter table a alter column aa type integer using bit_length(aa);
 select * from d;
 
+-- The above verified that we can change the type of a multiply-inherited
+-- column; but we should reject that if any definition was inherited from
+-- an unrelated parent.
+create temp table parent1(f1 int, f2 int);
+create temp table parent2(f1 int, f3 bigint);
+create temp table childtab(f4 int) inherits(parent1, parent2);
+alter table parent1 alter column f1 type bigint;  -- fail, conflict w/parent2
+alter table parent1 alter column f2 type bigint;  -- ok
+
 -- check that oid column is handled properly during alter table inherit
 create table oid_parent (a int) with oids;