diff --git a/src/tests/e2e-test b/src/tests/e2e-test
index 7d92515..c9f3b71 100755
--- a/src/tests/e2e-test
+++ b/src/tests/e2e-test
@@ -244,7 +244,10 @@ for run in $server_tests; do
     # XXX this ignores $TMPDIR, because it results in an overlong pathname on OS X
     tmux_socket="/tmp/.tmux-mosh-test-$$"
     ln -fs "${tmux_socket}" "${test_dir}/tmux-socket"
-    ${tmux_stdin} tmux -f /dev/null -S "${tmux_socket}" -C new-session "${srcdir}/print-exitstatus ${client_wrapper} ${sut} ${server_wrapper} \"${PWD}/${test_dir}/${run}\" \"${PWD}/${test_script} ${run}\"" > "${test_dir}/${run}.tmux.log"
+    # tmux <= 2.5 ignore -x/-y, but the client sets the session to 80x24.
+    # tmux from 2017-05-27 and later should default to an 80x24 session,
+    # but do use -x/-y on control-master clients.
+    ${tmux_stdin} tmux -f /dev/null -S "${tmux_socket}" -C new-session -x 80 -y 24 "${srcdir}/print-exitstatus ${client_wrapper} ${sut} ${server_wrapper} \"${PWD}/${test_dir}/${run}\" \"${PWD}/${test_script} ${run}\"" > "${test_dir}/${run}.tmux.log"
     rv=$?
     rm -f "${tmux_socket}" "${test_dir}/tmux-socket"
     if [ $rv -ne 0 ]; then