diff --git a/src/analyzer/protocol/http/HTTP.cc b/src/analyzer/protocol/http/HTTP.cc
index 318a3f9774..190e2def0a 100644
--- a/src/analyzer/protocol/http/HTTP.cc
+++ b/src/analyzer/protocol/http/HTTP.cc
@@ -987,6 +987,20 @@ void HTTP_Analyzer::DeliverStream(int len, const u_char* data, bool is_orig)
 						if ( request_method->ToStdString() != "GET" )
 							Weird("invalid_http_09_request_method", request_method->CheckString());
 
+						// If we already have a reply_message that means we saw
+						// an HTTP response before a request and interpreted
+						// it as HTTP/1.1 already. Reset the state here because
+						// we're removing the ContentLine support analyzer and
+						// any assumptions about expected delivery size state
+						// become invalid.
+						if ( reply_message )
+							{
+							Weird("http_09_reply_before_request");
+							reply_message->Done();
+							delete reply_message;
+							reply_message = nullptr;
+							}
+
 						reply_state = EXPECT_REPLY_HTTP09;
 						RemoveSupportAnalyzer(content_line_resp);
 						}
diff --git a/testing/btest/Baseline/scripts.base.protocols.http.http-09-content-length-confusion/http.log b/testing/btest/Baseline/scripts.base.protocols.http.http-09-content-length-confusion/http.log
new file mode 100644
index 0000000000..9bca082c7f
--- /dev/null
+++ b/testing/btest/Baseline/scripts.base.protocols.http.http-09-content-length-confusion/http.log
@@ -0,0 +1,12 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+#separator \x09
+#set_separator	,
+#empty_field	(empty)
+#unset_field	-
+#path	http
+#open XXXX-XX-XX-XX-XX-XX
+#fields	ts	uid	id.orig_h	id.orig_p	id.resp_h	id.resp_p	trans_depth	method	host	uri	referrer	version	user_agent	origin	request_body_len	response_body_len	status_code	status_msg	info_code	info_msg	tags	username	password	proxied	orig_fuids	orig_filenames	orig_mime_types	resp_fuids	resp_filenames	resp_mime_types
+#types	time	string	addr	port	addr	port	count	string	string	string	string	string	string	string	count	count	count	string	count	string	set[enum]	string	string	set[string]	vector[string]	vector[string]	vector[string]	vector[string]	vector[string]	vector[string]
+XXXXXXXXXX.XXXXXX	CHhAvVGS1DHFjwGM9	127.0.0.1	46982	127.0.0.1	80	1	GET	-	/	-	1.1	-	-	0	0	200	OK	-	-	(empty)	-	-	-	-	-	-	-	-	-
+XXXXXXXXXX.XXXXXX	CHhAvVGS1DHFjwGM9	127.0.0.1	46982	127.0.0.1	80	2	-	-	-	-	0.9	-	-	0	19	0	<empty>	-	-	(empty)	-	-	-	-	-	-	F7s8014qOBt9Gyy3w7	-	text/plain
+#close XXXX-XX-XX-XX-XX-XX
diff --git a/testing/btest/Baseline/scripts.base.protocols.http.http-09-content-length-confusion/weird.log b/testing/btest/Baseline/scripts.base.protocols.http.http-09-content-length-confusion/weird.log
new file mode 100644
index 0000000000..b6c643871a
--- /dev/null
+++ b/testing/btest/Baseline/scripts.base.protocols.http.http-09-content-length-confusion/weird.log
@@ -0,0 +1,12 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+#separator \x09
+#set_separator	,
+#empty_field	(empty)
+#unset_field	-
+#path	weird
+#open XXXX-XX-XX-XX-XX-XX
+#fields	ts	uid	id.orig_h	id.orig_p	id.resp_h	id.resp_p	name	addl	notice	peer	source
+#types	time	string	addr	port	addr	port	string	string	bool	string	string
+XXXXXXXXXX.XXXXXX	CHhAvVGS1DHFjwGM9	127.0.0.1	46982	127.0.0.1	80	http_09_reply_before_request	-	F	zeek	HTTP
+XXXXXXXXXX.XXXXXX	CHhAvVGS1DHFjwGM9	127.0.0.1	46982	127.0.0.1	80	HTTP_version_mismatch	-	F	zeek	HTTP
+#close XXXX-XX-XX-XX-XX-XX
diff --git a/testing/btest/Traces/http/http-09-content-length-confusion.pcap b/testing/btest/Traces/http/http-09-content-length-confusion.pcap
new file mode 100644
index 0000000000..f846fe2daf
Binary files /dev/null and b/testing/btest/Traces/http/http-09-content-length-confusion.pcap differ
diff --git a/testing/btest/scripts/base/protocols/http/http-09-content-length-confusion.zeek b/testing/btest/scripts/base/protocols/http/http-09-content-length-confusion.zeek
new file mode 100644
index 0000000000..10e16c3da2
--- /dev/null
+++ b/testing/btest/scripts/base/protocols/http/http-09-content-length-confusion.zeek
@@ -0,0 +1,7 @@
+# @TEST-DOC: HTTP response with Content-Length followed by HTTP/0.9 request. This triggered an assert.
+# @TEST-EXEC: zeek -b -Cr $TRACES/http/http-09-content-length-confusion.pcap %INPUT
+# @TEST-EXEC: btest-diff http.log
+# @TEST-EXEC: btest-diff weird.log
+
+@load base/frameworks/notice/weird
+@load base/protocols/http