diff --git a/src/thread.cpp b/src/thread.cpp
index 16ebd66..7004578 100644
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -25,7 +25,7 @@ namespace sta
 	void RtosThread::start()
 	{
 		// Check if there is no instance that is currently running.
-		STA_ASSERT(instance_ != NULL);
+		STA_ASSERT(instance_ == NULL);
 
 		instance_ = osThreadNew(entry_point, this, &attribs_);
 		STA_ASSERT(instance_ != NULL);