diff --git a/debian/matrix-synapse-py3.config b/debian/matrix-synapse-py3.config

index 3bda3292f1..37a781b3e8 100755
--- a/debian/matrix-synapse-py3.config
+++ b/debian/matrix-synapse-py3.config
@@ -5,7 +5,11 @@ set -e
 . /usr/share/debconf/confmodule
 
 # try to update the debconf db according to whatever is in the config files
-/opt/venvs/matrix-synapse/lib/manage_debconf.pl read || true
+#
+# note that we may get run during preconfiguration, in which case the script
+# will not yet be installed.
+[ -x /opt/venvs/matrix-synapse/lib/manage_debconf.pl ] && \
+    /opt/venvs/matrix-synapse/lib/manage_debconf.pl read
 
 db_input high matrix-synapse/server-name || true
 db_input high matrix-synapse/report-stats || true