diff --git a/ext/standard/http.c b/ext/standard/http.c
index 92d0af5cd9b..1145329a794 100644
--- a/ext/standard/http.c
+++ b/ext/standard/http.c
@@ -226,7 +226,7 @@ PHP_FUNCTION(http_build_query)
 		Z_PARAM_ARRAY_OR_OBJECT(formdata)
 		Z_PARAM_OPTIONAL
 		Z_PARAM_STRING(prefix, prefix_len)
-		Z_PARAM_STR(arg_sep)
+		Z_PARAM_STR_OR_NULL(arg_sep)
 		Z_PARAM_LONG(enc_type)
 	ZEND_PARSE_PARAMETERS_END();
 
diff --git a/ext/standard/tests/http/http_build_query/gh12745.phpt b/ext/standard/tests/http/http_build_query/gh12745.phpt
new file mode 100644
index 00000000000..3418498bf22
--- /dev/null
+++ b/ext/standard/tests/http/http_build_query/gh12745.phpt
@@ -0,0 +1,17 @@
+--TEST--
+Bug GH-9316 (http_build_query() default null argument for $arg_separator is implicitly coerced to string)
+--FILE--
+<?php
+
+$data = [
+    'hello' => 'world',
+    'foo' => 'bar',
+];
+
+var_dump(http_build_query(
+    $data,
+    encoding_type: PHP_QUERY_RFC3986
+));
+?>
+--EXPECT--
+string(19) "hello=world&foo=bar"