diff --git a/harness/gc.rb b/harness/gc.rb
index d3138aaa..06896d42 100644
--- a/harness/gc.rb
+++ b/harness/gc.rb
@@ -125,13 +125,13 @@ def run_benchmark(_num_itrs_hint, **, &block)
 
     if has_marking
       mark_bench = marking_times[bench_range]
-      avg_mark = mark_bench.sum / mark_bench.size
+      avg_mark = mark_bench.sum.fdiv(mark_bench.size)
       puts "Average marking time: %.1fms" % avg_mark
     end
 
     if has_sweeping
       sweep_bench = sweeping_times[bench_range]
-      avg_sweep = sweep_bench.sum / sweep_bench.size
+      avg_sweep = sweep_bench.sum.fdiv(sweep_bench.size)
       puts "Average sweeping time: %.1fms" % avg_sweep
     end
   end
diff --git a/test/gc_harness_test.rb b/test/gc_harness_test.rb
new file mode 100644
index 00000000..7906ff36
--- /dev/null
+++ b/test/gc_harness_test.rb
@@ -0,0 +1,32 @@
+require_relative 'test_helper'
+require 'open3'
+require 'tmpdir'
+require 'rbconfig'
+
+describe 'GC harness' do
+  # GC.stat reports marking and sweeping time in whole milliseconds, so the
+  # per-iteration deltas are Integers. Stub them to 1, 0, 0 and 3, 2, 2.
+  WORKLOAD = <<~'RUBY'
+    marks = [1, 0, 0]
+    sweeps = [3, 2, 2]
+    original = GCStats.method(:delta)
+    GCStats.define_singleton_method(:delta) do |before, after|
+      original.call(before, after).merge("gc_marking_time" => marks.shift || 0, "gc_sweeping_time" => sweeps.shift || 0)
+    end
+    run_benchmark(1) {}
+  RUBY
+
+  it 'prints average marking and sweeping times without truncating them' do
+    skip 'GC.stat has no marking_time on this Ruby' unless GC.stat.key?(:marking_time) && GC.stat.key?(:sweeping_time)
+
+    Dir.mktmpdir do |dir|
+      env = { 'WARMUP_ITRS' => '0', 'MIN_BENCH_ITRS' => '3', 'MIN_BENCH_TIME' => '0', 'RESULT_JSON_PATH' => File.join(dir, 'out.json') }
+      harness = File.expand_path('../harness/gc', __dir__)
+      stdout, stderr, status = Open3.capture3(env, RbConfig.ruby, '-r', harness, '-e', WORKLOAD)
+
+      assert status.success?, stderr
+      assert_includes stdout, 'Average marking time: 0.3ms'
+      assert_includes stdout, 'Average sweeping time: 2.3ms'
+    end
+  end
+end