diff --git a/ext/standard/tests/general_functions/gh21018.phpt b/ext/standard/tests/general_functions/gh21018.phpt
new file mode 100644
index 0000000000000..249cfb515aeb6
--- /dev/null
+++ b/ext/standard/tests/general_functions/gh21018.phpt
@@ -0,0 +1,21 @@
+--TEST--
+GH-21018 (header() removes headers with the same prefix)
+--INI--
+expose_php=On
+--CGI--
+--FILE--
+<?php
+header('a: 1');
+header('a-test: 1');
+header('a: 1');
+var_dump(headers_list());
+?>
+--EXPECTF--
+array(3) {
+  [0]=>
+  string(%d) "X-Powered-By: PHP/%s"
+  [1]=>
+  string(9) "a-test: 1"
+  [2]=>
+  string(4) "a: 1"
+}
diff --git a/main/SAPI.c b/main/SAPI.c
index 6709d467e34fe..c369be10ab71c 100644
--- a/main/SAPI.c
+++ b/main/SAPI.c
@@ -610,8 +610,12 @@ static void sapi_remove_header(zend_llist *l, char *name, size_t len, size_t hea
 	while (current) {
 		header = (sapi_header_struct *)(current->data);
 		next = current->next;
-		if (header->header_len > header_len
-				&& (header->header[header_len] == ':' || len > header_len)
+		/*
+		 * header_len is set for DELETE_PREFIX (used in cookies)
+		 * look for the : otherwise
+		 */
+		if (header->header_len > len
+				&& (header->header[len] == ':' || (header_len && len > header_len))
 				&& !strncasecmp(header->header, name, len)) {
 			if (current->prev) {
 				current->prev->next = next;