diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index bf913468d5..9c9d202eda 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -262,8 +262,7 @@ static void prim_abort(EvalState & state, Value * * args, Value & v)
 static void prim_throw(EvalState & state, Value * * args, Value & v)
 {
     PathSet context;
-    throw ThrownError(format("user-thrown exception: %1%") %
-        state.coerceToString(*args[0], context));
+    throw ThrownError(format("%1%") % state.coerceToString(*args[0], context));
 }