diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8661f324..7a72bc73 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@
 ### Fixed
 
 - Correctly set mouse events buttons property [#509]
+- Prevent 'Hash#[]=': can't add a new key into hash during iteration [#513]
 
 ### Removed
 
diff --git a/lib/ferrum/client/subscriber.rb b/lib/ferrum/client/subscriber.rb
index 74b8887c..ed720b57 100644
--- a/lib/ferrum/client/subscriber.rb
+++ b/lib/ferrum/client/subscriber.rb
@@ -8,7 +8,7 @@ class Subscriber
       def initialize
         @regular = Queue.new
         @priority = Queue.new
-        @on = Concurrent::Hash.new { |h, k| h[k] = Concurrent::Array.new }
+        @on = Concurrent::Hash.new
 
         start
       end
@@ -22,6 +22,7 @@ def <<(message)
       end
 
       def on(event, &block)
+        @on[event] ||= Concurrent::Array.new
         @on[event] << block
         true
       end
@@ -65,8 +66,8 @@ def call(message)
         method, session_id, params = message.values_at("method", "sessionId", "params")
         event = SessionClient.event_name(method, session_id)
 
-        total = @on[event].size
-        @on[event].each_with_index do |block, index|
+        total = @on[event]&.size.to_i
+        @on[event]&.each_with_index do |block, index|
           # In case of multiple callbacks we provide current index and total
           block.call(params, index, total)
         end