diff --git a/test/lib/helper.rb b/test/lib/helper.rb
index fe11727d..e52a1e19 100644
--- a/test/lib/helper.rb
+++ b/test/lib/helper.rb
@@ -224,11 +224,24 @@ def assert_local_raise(expected, message = nil)
     else
       assert_raise(expected, &block)
     end
-    stack = caller
-    assert_equal stack, error.backtrace&.last(stack.size)
+    assert_local_backtrace error
     error
   end
 
+  # Asserts that +error+ was raised in the same thread as +caller+ _and_ was
+  # called from the same level as +caller+.  The caller's own frame is ignored,
+  # as are all extra frames in +error+, but the remaining frames much match.
+  #
+  # NOTE: `stack = caller(2)` is different from `$!.backtrace[2..]` in JRuby.
+  # Rather than use `caller`, this raises a local exception to use its backtrace
+  # for the comparison.
+  def assert_local_backtrace(error)
+    local_stack = raise "generating local backtrace" rescue $!.backtrace[2..]
+    error_stack = error.backtrace&.last(local_stack.size)
+    assert_equal local_stack, error_stack
+    error_stack
+  end
+
   # Combines +assert_local_raise+ with an assertion that the exception's cause
   # is in the receiver thread.
   #
diff --git a/test/net/imap/test_imap_tls.rb b/test/net/imap/test_imap_tls.rb
index 052c114e..0fbceed8 100644
--- a/test/net/imap/test_imap_tls.rb
+++ b/test/net/imap/test_imap_tls.rb
@@ -110,7 +110,7 @@ def test_starttls_unknown_ca
         imap
       end
       assert_kind_of(OpenSSL::SSL::SSLError, ex)
-      assert_equal (stack = caller), ex.backtrace&.last(stack.size)
+      assert_local_backtrace ex
       assert_equal false, imap.tls_verified?
       assert_equal({}, imap.ssl_ctx_params)
       assert_equal(nil, imap.ssl_ctx.ca_file)