diff --git a/pkg/sql/opt/optbuilder/testdata/union b/pkg/sql/opt/optbuilder/testdata/union
index 8da5e9163cd1..9625f180e023 100644
--- a/pkg/sql/opt/optbuilder/testdata/union
+++ b/pkg/sql/opt/optbuilder/testdata/union
@@ -1079,3 +1079,17 @@ sort
            ├── columns: t68702.a:7 b:8 c:9
            └── scan t68702
                 └── columns: t68702.a:7 b:8 c:9 rowid:10!null crdb_internal_mvcc_timestamp:11 tableoid:12
+
+
+# Regression test for #70831. Do not add non-existent casts to match types on
+# either side of a set operation.
+# NOTE: In Postgres this example successfully returns a result. Our tuple typing
+# and casting logic doesn't match Postgres's semantics exactly, so we return a
+# user error. See
+# https://github.com/cockroachdb/cockroach/issues/70831#issuecomment-929547297.
+build
+SELECT * FROM (VALUES (ARRAY[(true, NULL)])) AS v1
+EXCEPT ALL
+SELECT * FROM (VALUES (ARRAY[]::RECORD[])) AS v2
+----
+error (42804): EXCEPT types tuple{bool, unknown}[] and tuple[] cannot be matched
diff --git a/pkg/sql/opt/optbuilder/union.go b/pkg/sql/opt/optbuilder/union.go
index 4dcfb27da631..f34373096d28 100644
--- a/pkg/sql/opt/optbuilder/union.go
+++ b/pkg/sql/opt/optbuilder/union.go
@@ -153,15 +153,20 @@ func determineUnionType(left, right *types.T, clauseTag string) *types.T {
 	}
 
 	if left.Equivalent(right) {
-		// Do a best-effort attempt to determine which type is "larger".
-		if left.Width() > right.Width() {
-			return left
-		}
+		// In the default case, use the left type.
+		src, tgt := right, left
 		if left.Width() < right.Width() {
-			return right
+			// If the right type is "larger", use it.
+			src, tgt = left, right
 		}
-		// In other cases, use the left type.
-		return left
+		if _, ok := tree.LookupCastVolatility(src, tgt, nil /* sessionData */); !ok {
+			// Error if no cast exists from src to tgt.
+			panic(pgerror.Newf(
+				pgcode.DatatypeMismatch,
+				"%v types %s and %s cannot be matched", clauseTag, left, right,
+			))
+		}
+		return tgt
 	}
 	leftFam, rightFam := left.Family(), right.Family()
 
@@ -189,7 +194,7 @@ func determineUnionType(left, right *types.T, clauseTag string) *types.T {
 		return right
 	}
 
-	// TODO(radu): Postgres has more encompassing rules:
+	// TODO(#75103): Postgres has more encompassing rules:
 	// http://www.postgresql.org/docs/12/static/typeconv-union-case.html
 	panic(pgerror.Newf(
 		pgcode.DatatypeMismatch,
@@ -198,7 +203,9 @@ func determineUnionType(left, right *types.T, clauseTag string) *types.T {
 }
 
 // addCasts adds a projection to a scope, adding casts as necessary so that the
-// resulting columns have the given types.
+// resulting columns have the given types. This function assumes that there is a
+// valid cast from the column types in dst.cols to the corresponding types in
+// outTypes.
 func (b *Builder) addCasts(dst *scope, outTypes []*types.T) *scope {
 	expr := dst.expr.(memo.RelExpr)
 	dstCols := dst.cols
diff --git a/pkg/sql/opt/optbuilder/union_test.go b/pkg/sql/opt/optbuilder/union_test.go
index 112d5159e3a0..6f04e16e98f2 100644
--- a/pkg/sql/opt/optbuilder/union_test.go
+++ b/pkg/sql/opt/optbuilder/union_test.go
@@ -72,6 +72,12 @@ func TestUnionType(t *testing.T) {
 			right:    types.String,
 			expected: nil,
 		},
+		{
+			// Error.
+			left:     types.MakeArray(types.MakeTuple([]*types.T{types.Bool})),
+			right:    types.MakeArray(types.MakeTuple([]*types.T{types.Bool, types.Int})),
+			expected: nil,
+		},
 	}
 
 	for _, tc := range testCases {