diff --git a/README.md b/README.md
index fba0bd1..396cd6b 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,9 @@ Method names and functionalities are inspired by [Kotlin.Collections](https://ko
 ### 1.1 Notes
 
 * Requires PHP 7.1 and above.
+* Thread safety:
+  * Distinct objects: **safe**.
+  * Shared objects: **unsafe**.
 
 ## 2. Documentation
 
diff --git a/config.w32 b/config.w32
index 3aae4c6..813753e 100644
--- a/config.w32
+++ b/config.w32
@@ -1,5 +1,8 @@
 ARG_ENABLE("collections", "enable collections support", "no");
 
 if (PHP_COLLECTIONS != "no") {
-    EXTENSION("collections", "src/collections.c src/collections_me.c src/collections_methods.c", true);
+    SRC="src/collections.c \
+        src/collections_me.c \
+        src/collections_methods.c"
+    EXTENSION("collections", SRC, true, "/DZEND_ENABLE_STATIC_TSRMLS_CACHE=1");
 }