diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 0585905e0..e1774237b 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -3810,7 +3810,7 @@ void DerivationGoal::runChild()
                     throw Error("unsupported builtin function '%1%'", string(drv->builder, 8));
                 _exit(0);
             } catch (std::exception & e) {
-                writeFull(STDERR_FILENO, e.what() + "\n");
+                writeFull(STDERR_FILENO, e.what() + std::string("\n"));
                 _exit(1);
             }
         }