diff --git a/changelog.d/17390.misc b/changelog.d/17390.misc
new file mode 100644
index 0000000000..6a4e344c5c
--- /dev/null
+++ b/changelog.d/17390.misc
@@ -0,0 +1 @@
+Fix building debian packages on non-clean checkouts.
diff --git a/docker/build_debian.sh b/docker/build_debian.sh
index 9eae38af91..00e0856c7d 100644
--- a/docker/build_debian.sh
+++ b/docker/build_debian.sh
@@ -11,6 +11,9 @@ DIST=$(cut -d ':' -f2 <<< "${distro:?}")
 cp -aT /synapse/source /synapse/build
 cd /synapse/build
 
+# Delete any existing `.so` files to ensure a clean build.
+rm -f /synapse/build/synapse/*.so
+
 # if this is a prerelease, set the Section accordingly.
 #
 # When the package is later added to the package repo, reprepro will use the