diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 9a04432e0..f56fd2d77 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -2079,7 +2079,7 @@ void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
             else if (n == "name")
                 request.name = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
-                throw EvalError(format("unsupported argument '%1%' to '%2%', at %3%") % attr.name % who % attr.pos);
+                throw EvalError(format("unsupported argument '%1%' to '%2%', at %3%") % attr.name % who % *attr.pos);
         }
 
         if (request.uri.empty())