diff --git a/bin/install-site.sh b/bin/install-site.sh
index 7332d6e..82885c7 100755
--- a/bin/install-site.sh
+++ b/bin/install-site.sh
@@ -157,7 +157,8 @@ update_apt_sources() {
                 notice_msg "Unsupported distribution and version combination $DISTRIBUTION $DISTVERSION"
                 ;;
         esac
-        # Install the basic packages we require:
+        # Install the basic packages we require, backing up any old sources.list
+        [ -f /etc/apt/sources.list ] && mv /etc/apt/sources.list /etc/apt/sources.list.dist
         cat > /etc/apt/sources.list.d/debian.sources <<EOF
 # Debian mirror to use, including contrib and non-free:
 Types: deb deb-src
diff --git a/shlib/installfns b/shlib/installfns
index cf11af8..2cac618 100755
--- a/shlib/installfns
+++ b/shlib/installfns
@@ -157,7 +157,8 @@ update_apt_sources() {
                 notice_msg "Unsupported distribution and version combination $DISTRIBUTION $DISTVERSION"
                 ;;
         esac
-        # Install the basic packages we require:
+        # Install the basic packages we require, backing up any old sources.list
+        [ -f /etc/apt/sources.list ] && mv /etc/apt/sources.list /etc/apt/sources.list.dist
         cat > /etc/apt/sources.list.d/debian.sources <<EOF
 # Debian mirror to use, including contrib and non-free:
 Types: deb deb-src