--- a/xpcom/glue/nsISupportsImpl.h
+++ b/xpcom/glue/nsISupportsImpl.h
@@ -125,19 +125,21 @@ do {                                    
 
 #define MOZ_LOG_DTOR(_ptr, _name, _size) \
 do {                                     \
   NS_LogDtor((void*)_ptr, _name, _size); \
 } while (0)
 
 /* nsCOMPtr.h allows these macros to be defined by clients
  * These logging functions require dynamic_cast<void*>, so they don't
- * do anything useful if we don't have dynamic_cast<void*>. */
+ * do anything useful if we don't have dynamic_cast<void*>.
+ * Note: The explicit comparison to nullptr is needed to avoid warnings
+ *       when _p is a nullptr itself. */
 #define NSCAP_LOG_ASSIGNMENT(_c, _p)                                \
-  if (_p)                                                           \
+  if (_p != nullptr)                                                \
     NS_LogCOMPtrAddRef((_c),static_cast<nsISupports*>(_p))
 
 #define NSCAP_LOG_RELEASE(_c, _p)                                   \
   if (_p)                                                           \
     NS_LogCOMPtrRelease((_c), static_cast<nsISupports*>(_p))
 
 #else /* !NS_BUILD_REFCNT_LOGGING */