diff --git a/core/src/avm2/globals/string.rs b/core/src/avm2/globals/string.rs
index 8b68592025f7..cd0664f5c9da 100644
--- a/core/src/avm2/globals/string.rs
+++ b/core/src/avm2/globals/string.rs
@@ -201,7 +201,7 @@ fn index_of<'gc>(
 ) -> Result<Value<'gc>, Error<'gc>> {
     let this = Value::from(this).coerce_to_string(activation)?;
     let pattern = match args.get(0) {
-        None => return Ok(Value::Undefined),
+        None => return Ok(Value::Integer(-1)),
         Some(s) => s.clone().coerce_to_string(activation)?,
     };
 
diff --git a/tests/tests/swfs/from_avmplus/ecma3/String/e15_5_4_6_1/test.toml b/tests/tests/swfs/from_avmplus/ecma3/String/e15_5_4_6_1/test.toml
index 29f3cef79022..cf6123969a1d 100644
--- a/tests/tests/swfs/from_avmplus/ecma3/String/e15_5_4_6_1/test.toml
+++ b/tests/tests/swfs/from_avmplus/ecma3/String/e15_5_4_6_1/test.toml
@@ -1,2 +1 @@
 num_ticks = 1
-known_failure = true