diff --git a/src/posix.cpp b/src/posix.cpp
index 62d03c83a7..c8e887b105 100644
--- a/src/posix.cpp
+++ b/src/posix.cpp
@@ -581,6 +581,7 @@ class MySystem: public System {
   }
 
   virtual void dispose() {
+    assert(this, count == 0);
     pthread_mutex_destroy(&mutex);
     ::free(this);
   }
diff --git a/src/windows.cpp b/src/windows.cpp
index c06cf27f7d..59c819eb87 100644
--- a/src/windows.cpp
+++ b/src/windows.cpp
@@ -627,6 +627,7 @@ class MySystem: public System {
   }
 
   virtual void dispose() {
+    assert(this, count == 0);
     CloseHandle(mutex);
     ::free(this);
   }