diff --git a/CHANGELOG.markdown b/CHANGELOG.markdown
index 463a3113..e3edaf17 100644
--- a/CHANGELOG.markdown
+++ b/CHANGELOG.markdown
@@ -4,6 +4,8 @@
 
 * Add support for Oxipng [#167](https://github.com/toy/image_optim/issues/167) [#190](https://github.com/toy/image_optim/pull/190) [@oblakeerickson](https://github.com/oblakeerickson)
 
+* Fix `TypeError: can't convert ImageOptim::Timer into Float` with Ruby 3 [#194](https://github.com/toy/image_optim/pull/194) [@yahonda](https://github.com/yahonda)
+
 ## v0.30.0 (2021-05-11)
 
 * Add `timeout` option to restrict maximum time spent on every image [#21](https://github.com/toy/image_optim/issues/21) [#148](https://github.com/toy/image_optim/pull/148) [#149](https://github.com/toy/image_optim/pull/149) [#162](https://github.com/toy/image_optim/pull/162) [#184](https://github.com/toy/image_optim/pull/184) [#189](https://github.com/toy/image_optim/pull/189) [@tgxworld](https://github.com/tgxworld) [@oblakeerickson](https://github.com/oblakeerickson) [@toy](https://github.com/toy)
diff --git a/lib/image_optim/cmd.rb b/lib/image_optim/cmd.rb
index ea00a282..4957f8d4 100644
--- a/lib/image_optim/cmd.rb
+++ b/lib/image_optim/cmd.rb
@@ -17,7 +17,7 @@ def run(*args)
           pid = Process.spawn(*args)
 
           waiter = Process.detach(pid)
-          if waiter.join(timeout)
+          if waiter.join(timeout.to_f)
             status = waiter.value
 
             check_status!(status)
diff --git a/spec/image_optim/cmd_spec.rb b/spec/image_optim/cmd_spec.rb
index 090784cd..44a94d1e 100644
--- a/spec/image_optim/cmd_spec.rb
+++ b/spec/image_optim/cmd_spec.rb
@@ -2,6 +2,7 @@
 
 require 'spec_helper'
 require 'image_optim/cmd'
+require 'image_optim/timer'
 
 describe ImageOptim::Cmd do
   before do
@@ -48,6 +49,11 @@ def expect_int_exception(&block)
         expect(Cmd.run('sh -c "exit 66"', timeout: 1)).to eq(false)
       end
 
+      it 'returns process success status when timeout is instance of ImageOptim::Timer' do
+        timeout = ImageOptim::Timer.new(1.0)
+        expect(Cmd.run('sh -c "exit 0"', timeout: timeout)).to eq(true)
+      end
+
       it 'raises SignalException if process terminates after signal', skip: SkipConditions[:signals_support] do
         expect_int_exception do
           Cmd.run('kill -s INT $$', timeout: 1)