diff --git a/src/V3HierBlock.cpp b/src/V3HierBlock.cpp
index 0f460f46d..f1858cdf0 100644
--- a/src/V3HierBlock.cpp
+++ b/src/V3HierBlock.cpp
@@ -202,7 +202,7 @@ string V3HierBlock::hierGenerated(bool withDir) const {
 
 string V3HierBlock::vFileIfNecessary() const {
     const string filename = V3Os::filenameRealPath(m_modp->fileline()->filename());
-    for (const string v : v3Global.opt.vFiles()) {
+    for (const string& v : v3Global.opt.vFiles()) {
         // Already listed in vFiles, so no need to add the file.
         if (filename == V3Os::filenameRealPath(v)) return "";
     }