diff --git a/src/HttpMsg.h b/src/HttpMsg.h
index 34f7a624fbb..1694f136a93 100644
--- a/src/HttpMsg.h
+++ b/src/HttpMsg.h
@@ -104,7 +104,7 @@ class HttpMsg : public RefCountable
 int httpMsgIsolateHeaders(const char **parse_start, int len, const char **blk_start, const char **blk_end);
 
 #define HTTPMSGUNLOCK(a) if (a) { if ((a)->unlock() == 0) delete (a); (a)=NULL; }
-#define HTTPMSGLOCK(a) (a)->lock()
+#define HTTPMSGLOCK(a) if (a) { (a)->lock(); }
 
 #endif /* SQUID_HTTPMSG_H */