diff --git a/lib/nusoap.php b/lib/nusoap.php
index f7be01f..ec3db1e 100644
--- a/lib/nusoap.php
+++ b/lib/nusoap.php
@@ -6176,7 +6176,11 @@ function serializeType($name, $type, $value, $use='encoded', $encodingStyle=fals
 				if ($forceType) {
 					$xml = "<$name$elementNS xsi:type=\"" . $this->getPrefixFromNamespace($ns) . ":$uqType\">$value</$name>";
 				} else {
-					$xml = "<$name$elementNS>$value</$name>";
+					if(is_null($value)) {
+						$xml = "<$name$elementNS xsi:nil=\"true\"/>";
+					} else {
+						$xml = "<$name$elementNS>$value</$name>";
+					}
 				}
 			} else {
 				$xml = "<$name$elementNS xsi:type=\"" . $this->getPrefixFromNamespace($ns) . ":$uqType\"$encodingStyle>$value</$name>";