diff --git a/NEWS b/NEWS
index 61114e8226a..869fa29a9b5 100644
--- a/NEWS
+++ b/NEWS
@@ -5,6 +5,7 @@ PHP                                                                        NEWS
 - Core:
   . Fixed bug GH-12953 (false positive SSA integrity verification failed when
     loading composer classmaps with more than 11k elements). (nielsdos)
+  . Fixed bug GH-12999 (zend_strnlen build when strnlen is unsupported). (rainerjung)
 
 - Cli:
   . Fix incorrect timeout in built-in web server when using router script and
diff --git a/Zend/zend_operators.h b/Zend/zend_operators.h
index 20eb7245afc..4f4a14b9305 100644
--- a/Zend/zend_operators.h
+++ b/Zend/zend_operators.h
@@ -269,7 +269,7 @@ static zend_always_inline size_t zend_strnlen(const char* s, size_t maxlen)
 #if defined(HAVE_STRNLEN)
 	return strnlen(s, maxlen);
 #else
-	const char *p = memchr(s, '\0', maxlen);
+	const char *p = (const char *)memchr(s, '\0', maxlen);
 	return p ? p-s : maxlen;
 #endif
 }