diff --git a/.gitlab-ci/run-tests.sh b/.gitlab-ci/run-tests.sh
index dc77342b9..63726d117 100755
--- a/.gitlab-ci/run-tests.sh
+++ b/.gitlab-ci/run-tests.sh
@@ -7,6 +7,8 @@ dconf update
 mkdir -m 700 $XDG_RUNTIME_DIR
 glib-compile-schemas $GSETTINGS_SCHEMA_DIR
 
+export MUTTER_DEBUG_DUMMY_MODE_SPECS="800x600@10.0"
+
 dbus-run-session -- \
     xvfb-run -s '+iglx -noreset' \
         meson test -C build --no-rebuild -t 10 --wrap catchsegv