diff --git a/src/lambda-calculus.js b/src/lambda-calculus.js
index e68604d..7479b76 100644
--- a/src/lambda-calculus.js
+++ b/src/lambda-calculus.js
@@ -42,7 +42,7 @@ class L {
     this.body = body;
   }
   free() {
-    const r = this.body.free?.() || new Set ;
+    const r = this.body?.free?.() || new Set ;
     r.delete(this.name);
     return r;
   }
@@ -58,7 +58,7 @@ class A {
     this.left = left;
     this.right = right;
   }
-  free() { return union( this.left.free?.() || [] , this.right.free?.() || [] ); }
+  free() { return union( this.left?.free?.() || [] , this.right?.free?.() || [] ); }
   toString() {
     const left = this.left instanceof L ? `(${this.left})` : this.left ;
     const right = this.right instanceof V ? this.right : `(${this.right})` ;
@@ -169,7 +169,7 @@ export function toInt(term) {
 function parse(code) {
   function parseTerm(env,code) {
     function wrap(name,term) {
-      const FV = term.free?.() || new Set ; FV.delete("()");
+      const FV = term?.free?.() || new Set ; FV.delete("()");
       if ( config.purity === "Let" )
         return Array.from(FV).reduce( (tm,nm) => {
           if ( env.has(nm) ) {