diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 0b6a214ef..27ba1c84a 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2748,8 +2748,18 @@ void DerivationGoal::registerOutputs()
         if (buildMode == bmCheck) {
             if (!store->isValidPath(path)) continue;
             ValidPathInfo info = worker.store.queryPathInfo(path);
-            if (hash.first != info.hash)
-                throw Error(format("derivation ‘%1%’ may not be deterministic: hash mismatch in output ‘%2%’") % drvPath % path);
+            if (hash.first != info.hash) {
+                if (settings.keepFailed) {
+                    Path dst = path + "-check";
+                    if (pathExists(dst)) deletePath(dst);
+                    if (rename(actualPath.c_str(), dst.c_str()))
+                        throw SysError(format("renaming ‘%1%’ to ‘%2%’") % actualPath % dst);
+                    throw Error(format("derivation ‘%1%’ may not be deterministic: output ‘%2%’ differs from ‘%3%’")
+                        % drvPath % path % dst);
+                } else
+                    throw Error(format("derivation ‘%1%’ may not be deterministic: output ‘%2%’ differs")
+                        % drvPath % path);
+            }
             continue;
         }