diff --git a/src/core/include/mp-units/format.h b/src/core/include/mp-units/format.h
index 8cc5f25e..cc158e74 100644
--- a/src/core/include/mp-units/format.h
+++ b/src/core/include/mp-units/format.h
@@ -452,7 +452,7 @@ class MP_UNITS_STD_FMT::formatter<mp_units::quantity<Reference, Rep>, Char> {
     }
     // user provided format
     quantity_formatter f{*this, out, q, locale};
-    parse_quantity_specs(modifiers_format_str_.begin(), modifiers_format_str_.end(), f);
+    parse_quantity_specs(modifiers_format_str_.data(), modifiers_format_str_.data() +  modifiers_format_str_.size(), f);
     return f.out;
   }