diff --git a/src/protocol/ast_printer.rs b/src/protocol/ast_printer.rs
index 0818cfbac700900528d25955f2820909bcaa9307..dd338abccd1d9a1dcaddf15254327f9f8320b7b9 100644
--- a/src/protocol/ast_printer.rs
+++ b/src/protocol/ast_printer.rs
@@ -599,7 +599,7 @@ impl ASTWriter {
                 self.kv(indent).with_id(PREFIX_BINARY_EXPR_ID, expr.this.0.index)
                     .with_s_key("BindingExpr");
                 self.kv(indent2).with_s_key("LeftExpression");
-                self.write_expr(heap, expr.left, indent3);
+                self.write_expr(heap, expr.left.upcast(), indent3);
                 self.kv(indent2).with_s_key("RightExpression");
                 self.write_expr(heap, expr.right, indent3);
                 self.kv(indent2).with_s_key("Parent")