diff --git a/c/tester.js b/c/tester.js
index 26922ad..9a15b2a 100644
--- a/c/tester.js
+++ b/c/tester.js
@@ -186,7 +186,7 @@ class CTester {
         if (!self.symbols) await self.loadSymbols();
         for (let n in self.symbols) {
             let v;
-            if (utils.isDefined(witness[self.symbols[n].varIdx])) {
+            if (isDefined(witness[self.symbols[n].varIdx])) {
                 v = witness[self.symbols[n].varIdx].toString();
             } else {
                 v = "undefined";
@@ -271,3 +271,7 @@ function fromArray8ToUint(arr) { //returns a BigInt
     }
     return res;
 }
+
+function isDefined(v) {
+    return ((typeof(v) != "undefined")&&(v !== null));
+}
diff --git a/wasm/tester.js b/wasm/tester.js
index 1609503..3331fcb 100644
--- a/wasm/tester.js
+++ b/wasm/tester.js
@@ -174,7 +174,7 @@ class WasmTester {
         if (!self.symbols) await self.loadSymbols();
         for (let n in self.symbols) {
             let v;
-            if (utils.isDefined(witness[self.symbols[n].varIdx])) {
+            if (isDefined(witness[self.symbols[n].varIdx])) {
                 v = witness[self.symbols[n].varIdx].toString();
             } else {
                 v = "undefined";
@@ -236,3 +236,7 @@ async function compiler_above_version(v) {
     vlist = version_to_list(v);
     return check_versions ( compiler_version, vlist );
 }
+
+function isDefined(v) {
+    return ((typeof(v) != "undefined")&&(v !== null));
+}
\ No newline at end of file