diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSString.java b/jso/apis/src/main/java/org/teavm/jso/core/JSString.java
index da4446f50..bca4ee593 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSString.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSString.java
@@ -99,6 +99,6 @@ public abstract class JSString implements JSObject {
 
     public abstract JSString trim();
 
-    @JSBody(params = "obj", script = "return typeof this === 'string';")
+    @JSBody(params = "obj", script = "return typeof obj === 'string';")
     public static native boolean isInstance(JSObject obj);
 }