diff --git a/src/posix/platform/system.cpp b/src/posix/platform/system.cpp
index 9e0018771..609d399fd 100644
--- a/src/posix/platform/system.cpp
+++ b/src/posix/platform/system.cpp
@@ -92,13 +92,13 @@ otInstance *otSysInit(otPlatformConfig *aPlatformConfig)
 #endif
     platformRandomInit();
 
-    instance = otInstanceInitSingle();
-    assert(instance != nullptr);
-
 #if OPENTHREAD_CONFIG_HEAP_EXTERNAL_ENABLE
     otHeapSetCAllocFree(calloc, free);
 #endif
 
+    instance = otInstanceInitSingle();
+    assert(instance != nullptr);
+
 #if OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
     platformBackboneInit(instance, aPlatformConfig->mBackboneInterfaceName);
 #endif