3 files changed, 7 insertions, 2 deletions
diff --git a/Libraries/LibJS/Runtime/ArrayPrototype.cpp b/Libraries/LibJS/Runtime/ArrayPrototype.cpp
index ecd14217c1..edfa930ed2 100644
--- a/Libraries/LibJS/Runtime/ArrayPrototype.cpp
+++ b/Libraries/LibJS/Runtime/ArrayPrototype.cpp
@@ -357,7 +357,7 @@ Value ArrayPrototype::index_of(Interpreter& interpreter)
         return {};
 
     i32 array_size = static_cast<i32>(array->elements().size());
-    if (interpreter.argument_count() == 0 || array_size == 0)
+    if (array_size == 0)
         return Value(-1);
 
     i32 from_index = 0;
@@ -411,7 +411,7 @@ Value ArrayPrototype::last_index_of(Interpreter& interpreter)
         return {};
 
     i32 array_size = static_cast<i32>(array->elements().size());
-    if (interpreter.argument_count() == 0 || array_size == 0)
+    if (array_size == 0)
         return Value(-1);
 
     i32 from_index = 0;
diff --git a/Libraries/LibJS/Tests/Array.prototype.indexOf.js b/Libraries/LibJS/Tests/Array.prototype.indexOf.js
index 630745c283..12cc3e7333 100644
--- a/Libraries/LibJS/Tests/Array.prototype.indexOf.js
+++ b/Libraries/LibJS/Tests/Array.prototype.indexOf.js
@@ -20,6 +20,8 @@ try {
     assert([].indexOf('serenity') === -1);
     assert([].indexOf('serenity', 10) === -1);
     assert([].indexOf('serenity', -10) === -1);
+    assert([].indexOf() === -1);
+    assert([undefined].indexOf() === 0);
 
     console.log("PASS");
 } catch (e) {
diff --git a/Libraries/LibJS/Tests/Array.prototype.lastIndexOf.js b/Libraries/LibJS/Tests/Array.prototype.lastIndexOf.js
index 642c897292..1ae10c4e55 100644
--- a/Libraries/LibJS/Tests/Array.prototype.lastIndexOf.js
+++ b/Libraries/LibJS/Tests/Array.prototype.lastIndexOf.js
@@ -15,6 +15,9 @@ try {
     assert([].lastIndexOf('hello') === -1);
     assert([].lastIndexOf('hello', 10) === -1);
     assert([].lastIndexOf('hello', -10) === -1);
+    assert([].lastIndexOf() === -1);
+    assert([undefined].lastIndexOf() === 0);
+    assert([undefined, undefined, undefined].lastIndexOf() === 2);
 
     console.log("PASS");
 } catch (e) {