diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index e75b5e38d9..df0d246ae3 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -5004,6 +5004,8 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 			if g.fn_decl.return_type.is_ptr() {
 				var_str := g.expr_string(expr0)
 				g.write(var_str.trim('&'))
+			} else if g.table.sym(g.fn_decl.return_type).kind in [.sum_type, .interface_] {
+				g.expr_with_cast(expr0, node.types[0], g.fn_decl.return_type)
 			} else {
 				g.write('*')
 				g.expr(expr0)
diff --git a/vlib/v/tests/fn_return_mut_sumtype_test.v b/vlib/v/tests/fn_return_mut_sumtype_test.v
new file mode 100644
index 0000000000..a529611e5a
--- /dev/null
+++ b/vlib/v/tests/fn_return_mut_sumtype_test.v
@@ -0,0 +1,23 @@
+type Sum = Struct | int
+
+struct Struct {
+mut:
+	value int
+}
+
+fn update(mut s Struct) Sum {
+	s.value += 1
+	return s
+}
+
+fn test_fn_return_mut_sumtype() {
+	mut s := Sum(Struct{
+		value: 1
+	})
+	if mut s is Struct {
+		s = update(mut s)
+		assert s.value == 2
+	} else {
+		assert false
+	}
+}