diff --git a/src/libstore/unix/build/derivation-builder.cc b/src/libstore/unix/build/derivation-builder.cc
index ff06acfbb..6baf61125 100644
--- a/src/libstore/unix/build/derivation-builder.cc
+++ b/src/libstore/unix/build/derivation-builder.cc
@@ -1016,7 +1016,7 @@ void DerivationBuilderImpl::processSandboxSetupMessages()
                 e.addTrace({}, "while waiting for the build environment for '%s' to initialize (%s, previous messages: %s)",
                     store.printStorePath(drvPath),
                     statusToString(status),
-                    concatStringsSep("|", msgs));
+                    concatStringsSep("\n", msgs));
                 throw;
             }
         }();