set read only property properly (#11761)

diff --git a/erpnext/public/js/controllers/transaction.js b/erpnext/public/js/controllers/transaction.js
index b07d090..cb1cf3a 100644
--- a/erpnext/public/js/controllers/transaction.js
+++ b/erpnext/public/js/controllers/transaction.js
@@ -550,7 +550,7 @@
 
 		}
 		// Make read only if Accounts Settings doesn't allow stale rates
-		this.frm.set_df_property("conversion_rate", "read_only", erpnext.stale_rate_allowed());
+		this.frm.set_df_property("conversion_rate", "read_only", erpnext.stale_rate_allowed() ? 0 : 1);
 	},
 
 	set_actual_charges_based_on_currency: function() {