diff --git a/core/fiber.rbs b/core/fiber.rbs
index f9d7553ab..151ad2225 100644
--- a/core/fiber.rbs
+++ b/core/fiber.rbs
@@ -86,7 +86,7 @@ class Fiber < Object
   #
   # See also Fiber::[]=.
   #
-  def self.[]: (Symbol | String) -> untyped
+  def self.[]: (Symbol) -> untyped
 
   # <!--
   #   rdoc-file=cont.c
@@ -99,7 +99,7 @@ class Fiber < Object
   #
   # See also Fiber::[].
   #
-  def self.[]=: [A] (Symbol | String, A) -> A
+  def self.[]=: [A] (Symbol, A) -> A
 
   # <!--
   #   rdoc-file=cont.c
diff --git a/test/stdlib/Fiber_test.rb b/test/stdlib/Fiber_test.rb
index 7ced775fc..2cdadf03d 100644
--- a/test/stdlib/Fiber_test.rb
+++ b/test/stdlib/Fiber_test.rb
@@ -15,18 +15,6 @@ def test_aref
       "(Symbol) -> Integer",
       Fiber, :[], :key
     )
-
-    key = "string"
-
-    assert_send_type(
-      "(String, Integer) -> Integer",
-      Fiber, :[]=, key, 123
-    )
-
-    assert_send_type(
-      "(String) -> Integer",
-      Fiber, :[], key
-    )
   end
 
   def test_blocking?