diff --git a/for-js/Velopack.js b/for-js/Velopack.js
index c0c7b37..f21f6b7 100644
--- a/for-js/Velopack.js
+++ b/for-js/Velopack.js
@@ -1200,9 +1200,11 @@ class UpdateManager extends UpdateManagerSync {
     async downloadUpdatesAsync(toDownload, progress) {
         const command = this.getDownloadUpdatesCommand(toDownload);
         await nativeStartProcessAsyncReadLine(command, (data) => {
-            const p = parseInt(data);
-            if (!isNaN(p) && p > 0) {
-                progress(p);
+            if (progress && progress instanceof Function) {
+                const p = parseInt(data);
+                if (!isNaN(p) && p > 0) {
+                    progress(p);
+                }
             }
         });
     }
diff --git a/for-js/Velopack.ts b/for-js/Velopack.ts
index 3cf8638..50b159e 100644
--- a/for-js/Velopack.ts
+++ b/for-js/Velopack.ts
@@ -1331,9 +1331,11 @@ export class UpdateManager extends UpdateManagerSync {
   ): Promise<void> {
     const command: string[] = this.getDownloadUpdatesCommand(toDownload);
     await nativeStartProcessAsyncReadLine(command, (data: string) => {
-      const p = parseInt(data);
-      if (!isNaN(p) && p > 0) {
-        progress(p);
+      if (progress && progress instanceof Function) {
+        const p = parseInt(data);
+        if (!isNaN(p) && p > 0) {
+          progress(p);
+        }
       }
     });
   }
diff --git a/src/include/ts_end.ts b/src/include/ts_end.ts
index f435f46..b34a054 100644
--- a/src/include/ts_end.ts
+++ b/src/include/ts_end.ts
@@ -36,9 +36,11 @@ export class UpdateManager extends UpdateManagerSync {
     public async downloadUpdatesAsync(toDownload: VelopackAsset, progress: (arg: number) => void): Promise<void> {
         const command: string[] = this.getDownloadUpdatesCommand(toDownload);
         await nativeStartProcessAsyncReadLine(command, (data: string) => {
-            const p = parseInt(data);
-            if (!isNaN(p) && p > 0) {
-                progress(p);
+            if (progress && progress instanceof Function) {
+                const p = parseInt(data);
+                if (!isNaN(p) && p > 0) {
+                    progress(p);
+                }
             }
         });
     }