diff --git a/include/pybind11/numpy.h b/include/pybind11/numpy.h
index 04001d6c5..588cc1519 100644
--- a/include/pybind11/numpy.h
+++ b/include/pybind11/numpy.h
@@ -170,7 +170,7 @@ public:
         m_ptr = descr.strip_padding().release().ptr();
     }
 
-    explicit dtype(std::string format) {
+    explicit dtype(const std::string &format) {
         m_ptr = from_args(pybind11::str(format)).release().ptr();
     }