diff --git a/tool/lib/core_assertions.rb b/tool/lib/core_assertions.rb
index 419704448b..e29a0e3c25 100644
--- a/tool/lib/core_assertions.rb
+++ b/tool/lib/core_assertions.rb
@@ -384,11 +384,10 @@ eom
         end
         raise if $!
         abort = status.coredump? || (status.signaled? && ABORT_SIGNALS.include?(status.termsig))
-        assertions = 0
         marshal_error = nil
         assert(!abort, FailDesc[status, nil, stderr])
         res.scan(/^<error id="#{token_re}" assertions=(\d+)>\n(.*?)\n(?=<\/error id="#{token_re}">$)/m) do
-          assertions += $1.to_i
+          self._assertions += $1.to_i
           res = Marshal.load($2.unpack1("m")) or next
         rescue => marshal_error
           ignore_stderr = nil
diff --git a/tool/test/testunit/test_assertion.rb b/tool/test/testunit/test_assertion.rb
index 7b1f28a869..d9bdc8f3c5 100644
--- a/tool/test/testunit/test_assertion.rb
+++ b/tool/test/testunit/test_assertion.rb
@@ -17,6 +17,29 @@ class TestAssertion < Test::Unit::TestCase
     end
   end
 
+  def test_assertion_count_separately
+    beginning = self._assertions
+
+    assert_separately([], "")
+    assertions_at_nothing = self._assertions - beginning
+
+    prev_assertions = self._assertions + assertions_at_nothing
+    assert_separately([], "assert true")
+    assert_equal(1, self._assertions - prev_assertions)
+
+    omit unless Process.respond_to?(:fork)
+    prev_assertions = self._assertions + assertions_at_nothing
+    assert_separately([], "Process.fork {assert true}; assert true")
+    assert_equal(2, self._assertions - prev_assertions)
+
+    prev_assertions = self._assertions + assertions_at_nothing
+    # TODO: assertions before `fork` are counted twice; it is possible
+    # to reset `_assertions` at `Process._fork`, but the hook can
+    # interfere in other tests.
+    assert_separately([], "assert true; Process.fork {assert true}")
+    assert_equal(3, self._assertions - prev_assertions)
+  end
+
   def return_in_assert_raise
     assert_raise(RuntimeError) do
       return