diff --git a/ext/soap/php_encoding.c b/ext/soap/php_encoding.c
index 0b4b553d87540..2e03eac0f1b29 100644
--- a/ext/soap/php_encoding.c
+++ b/ext/soap/php_encoding.c
@@ -3584,6 +3584,11 @@ static encodePtr get_array_type(xmlNodePtr node, zval *array, smart_str *type)
 				soap_error0(E_ERROR, "Encoding: SoapVar has no 'enc_type' property");
 			}
 			cur_type = Z_LVAL_P(ztype);
+			if (cur_type == UNKNOWN_TYPE) {
+				/* Mimic guess_xml_convert() where we use the type of the data.
+				 * UNDEFs are handled transparently as it will error out upon encoding the data. */
+				cur_type = Z_TYPE_P(Z_VAR_ENC_VALUE_P(tmp));
+			}
 
 			zval *zstype = Z_VAR_ENC_STYPE_P(tmp);
 			if (Z_TYPE_P(zstype) == IS_STRING) {
diff --git a/ext/soap/tests/bugs/gh20011.phpt b/ext/soap/tests/bugs/gh20011.phpt
new file mode 100644
index 0000000000000..cf40d813e6178
--- /dev/null
+++ b/ext/soap/tests/bugs/gh20011.phpt
@@ -0,0 +1,20 @@
+--TEST--
+GH-20011 (Array of SoapVar of unknown type causes crash)
+--EXTENSIONS--
+soap
+--FILE--
+<?php +class TestSoapClient extends SoapClient { + public function __doRequest($request, $location, $action, $version, $one_way = false): ?string { + die($request); + } +} + +$array = [new SoapVar('test string', NULL)]; + +$client = new TestSoapClient(NULL, ['location' => 'test://', 'uri' => 'http://soapinterop.org/']);
+$client->echoStringArray($array);
+?>
+--EXPECT--
+<?xml version="1.0" encoding="UTF-8"?>
+<soap-env:envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ns1="http://soapinterop.org/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" SOAP-ENV:encoding><soap-env:body><ns1:echostringarray><param0 SOAP-ENC:arrayType="xsd:string[1]" xsi:type="SOAP-ENC:Array"><item xsi:type="xsd:string">test string</item></param0></ns1:echostringarray></soap-env:body></soap-env:envelope>
</div><div class="naked_ctrl">
<form action="/index.cgi/contrast" method="get" name="gate">
<p><a href="http://altstyle.alfasado.net">AltStyle</a> によって変換されたページ <a href="https://patch-diff.githubusercontent.com/raw/php/php-src/pull/20030.diff">(-&gt;オリジナル)</a>
/ <label>アドレス: <input type="text" name="naked_post_url" value="https://patch-diff.githubusercontent.com/raw/php/php-src/pull/20030.diff" size="22" /></label> <label>モード: <select name="naked_post_mode">
<option value="default">デフォルト</option>
<option value="speech">音声ブラウザ</option>
<option value="ruby">ルビ付き</option>
<option value="contrast" selected="selected">配色反転</option>
<option value="larger-text">文字拡大</option>
<option value="mobile">モバイル</option>
</select>
<input type="submit" value="表示" />
</p>
</form>
</div>