diff --git a/Makefile b/Makefile
index 860730ba..cc8aa3f0 100644
--- a/Makefile
+++ b/Makefile
@@ -187,6 +187,13 @@ run:
 		-e DMS_DEBUG=0 \
 		-h mail.my-domain.com -t $(NAME)
 	sleep 20
+	docker run -d --name mail_undef_spam_subject \
+		-v "`pwd`/test/config":/tmp/docker-mailserver \
+		-v "`pwd`/test":/tmp/docker-mailserver-test \
+		-e ENABLE_SPAMASSASSIN=1 \
+		-e SA_SPAM_SUBJECT="undef" \
+		-h mail.my-domain.com -t $(NAME)
+	sleep 15
 
 
 generate-accounts-after-run:
@@ -246,6 +253,7 @@ clean:
 		mail_with_imap \
 		mail_lmtp_ip \
 		mail_with_postgrey \
+		mail_undef_spam_subject \
 		mail_postscreen \
 		mail_override_hostname
 
diff --git a/target/start-mailserver.sh b/target/start-mailserver.sh
index a7ee6209..a463f8df 100644
--- a/target/start-mailserver.sh
+++ b/target/start-mailserver.sh
@@ -1013,7 +1013,12 @@ function _setup_security_stack() {
 		SA_TAG=${SA_TAG:="2.0"} && sed -i -r 's/^\$sa_tag_level_deflt (.*);/\$sa_tag_level_deflt = '$SA_TAG';/g' /etc/amavis/conf.d/20-debian_defaults
 		SA_TAG2=${SA_TAG2:="6.31"} && sed -i -r 's/^\$sa_tag2_level_deflt (.*);/\$sa_tag2_level_deflt = '$SA_TAG2';/g' /etc/amavis/conf.d/20-debian_defaults
 		SA_KILL=${SA_KILL:="6.31"} && sed -i -r 's/^\$sa_kill_level_deflt (.*);/\$sa_kill_level_deflt = '$SA_KILL';/g' /etc/amavis/conf.d/20-debian_defaults
-		SA_SPAM_SUBJECT=${SA_SPAM_SUBJECT:="***SPAM*** "} && sed -i -r 's/^\$sa_spam_subject_tag (.*);/\$sa_spam_subject_tag = '"'$SA_SPAM_SUBJECT'"';/g' /etc/amavis/conf.d/20-debian_defaults
+		SA_SPAM_SUBJECT=${SA_SPAM_SUBJECT:="***SPAM*** "}
+		if [ "$SA_SPAM_SUBJECT" == "undef" ]; then
+			sed -i -r 's/^\$sa_spam_subject_tag (.*);/\$sa_spam_subject_tag = undef;/g' /etc/amavis/conf.d/20-debian_defaults
+		else
+			sed -i -r 's/^\$sa_spam_subject_tag (.*);/\$sa_spam_subject_tag = '"'$SA_SPAM_SUBJECT'"';/g' /etc/amavis/conf.d/20-debian_defaults
+		fi
 		test -e /tmp/docker-mailserver/spamassassin-rules.cf && cp /tmp/docker-mailserver/spamassassin-rules.cf /etc/spamassassin/
 	fi
 
diff --git a/test/tests.bats b/test/tests.bats
index 5b129165..a7a1ebf6 100644
--- a/test/tests.bats
+++ b/test/tests.bats
@@ -474,6 +474,8 @@ load 'test_helper/bats-assert/load'
   assert_success
   run docker exec mail /bin/sh -c "grep '\$sa_spam_subject_tag' /etc/amavis/conf.d/20-debian_defaults | grep '= .SPAM: .'"
   assert_success
+  run docker exec mail_undef_spam_subject /bin/sh -c "grep '\$sa_spam_subject_tag' /etc/amavis/conf.d/20-debian_defaults | grep '= undef'"
+  assert_success
 }
 
 @test "checking spamassassin: all registered domains should see spam headers" {