diff --git a/vpnsetup_centos.sh b/vpnsetup_centos.sh
index 26b6a0f..6cb5eb4 100755
--- a/vpnsetup_centos.sh
+++ b/vpnsetup_centos.sh
@@ -578,7 +578,8 @@ net.ipv4.tcp_rmem = 4096 87380 16777216
 net.ipv4.tcp_wmem = 4096 87380 16777216
 EOF
     if modprobe -q tcp_bbr \
-      && printf '%s\n%s' "4.20" "$(uname -r)" | sort -C -V; then
+      && printf '%s\n%s' "4.20" "$(uname -r)" | sort -C -V \
+      && [ -f /proc/sys/net/ipv4/tcp_congestion_control ]; then
 cat >> /etc/sysctl.conf <<'EOF'
 net.core.default_qdisc = fq
 net.ipv4.tcp_congestion_control = bbr
diff --git a/vpnsetup_ubuntu.sh b/vpnsetup_ubuntu.sh
index 75f8afe..a8eea7c 100755
--- a/vpnsetup_ubuntu.sh
+++ b/vpnsetup_ubuntu.sh
@@ -559,7 +559,8 @@ net.ipv4.tcp_rmem = 4096 87380 16777216
 net.ipv4.tcp_wmem = 4096 87380 16777216
 EOF
     if modprobe -q tcp_bbr \
-      && printf '%s\n%s' "4.20" "$(uname -r)" | sort -C -V; then
+      && printf '%s\n%s' "4.20" "$(uname -r)" | sort -C -V \
+      && [ -f /proc/sys/net/ipv4/tcp_congestion_control ]; then
 cat >> /etc/sysctl.conf <<'EOF'
 net.core.default_qdisc = fq
 net.ipv4.tcp_congestion_control = bbr