diff --git a/Zend/zend_closures.c b/Zend/zend_closures.c
index f697f25a0f80..2cad01772245 100644
--- a/Zend/zend_closures.c
+++ b/Zend/zend_closures.c
@@ -696,7 +696,6 @@ static ZEND_NAMED_FUNCTION(zend_closure_internal_handler) /* {{{ */
 	zend_closure *closure = (zend_closure*)ZEND_CLOSURE_OBJECT(EX(func));
 	closure->orig_internal_handler(INTERNAL_FUNCTION_PARAM_PASSTHRU);
 	OBJ_RELEASE((zend_object*)closure);
-	EX(func) = NULL;
 }
 /* }}} */
 
diff --git a/Zend/zend_observer.c b/Zend/zend_observer.c
index f699ea714d83..79929bfdd80e 100644
--- a/Zend/zend_observer.c
+++ b/Zend/zend_observer.c
@@ -208,6 +208,7 @@ ZEND_API bool zend_observer_remove_end_handler(zend_function *function, zend_obs
 
 static inline zend_execute_data **prev_observed_frame(zend_execute_data *execute_data) {
 	zend_function *func = EX(func);
+	ZEND_ASSERT(func);
 	return (zend_execute_data **)&Z_PTR_P(EX_VAR_NUM((ZEND_USER_CODE(func->type) ? func->op_array.last_var : ZEND_CALL_NUM_ARGS(execute_data)) + func->common.T - 1));
 }
 
@@ -260,6 +261,7 @@ ZEND_API void ZEND_FASTCALL zend_observer_fcall_begin(zend_execute_data *execute
 
 static inline void call_end_observers(zend_execute_data *execute_data, zval *return_value) {
 	zend_function *func = execute_data->func;
+	ZEND_ASSERT(func);
 
 	zend_observer_fcall_end_handler *handler = (zend_observer_fcall_end_handler *)&ZEND_OBSERVER_DATA(func) + zend_observers_fcall_list.count;
 	// TODO: Fix exceptions from generators
diff --git a/ext/zend_test/tests/observer_closure_03.phpt b/ext/zend_test/tests/observer_closure_03.phpt
new file mode 100644
index 000000000000..247dfb361f57
--- /dev/null
+++ b/ext/zend_test/tests/observer_closure_03.phpt
@@ -0,0 +1,26 @@
+--TEST--
+Observer: Observability of closures of builtin functions
+--EXTENSIONS--
+zend_test
+--INI--
+zend_test.observer.enabled=1
+zend_test.observer.observe_all=1
+--FILE--
+<?php
+$closure = strlen(...);
+var_dump($closure('test'));
+
+echo 'DONE' . PHP_EOL;
+?>
+--EXPECTF--
+<!-- init '%s' -->
+<file '%s'>
+  <!-- init strlen() -->
+  <strlen>
+  </strlen>
+  <!-- init var_dump() -->
+  <var_dump>
+int(4)
+  </var_dump>
+DONE
+</file '%s'>