diff --git a/lix/libcmd/repl.cc b/lix/libcmd/repl.cc
index 203f9414d..899fdc4a8 100644
--- a/lix/libcmd/repl.cc
+++ b/lix/libcmd/repl.cc
@@ -713,6 +713,8 @@ ProcessLineResult NixRepl::processLine(std::string line)
     }
 
     else if (command == ":log") {
+        if (arg.empty())
+            throw Error("cannot use ':log' without a specifying a derivation");
         StorePath drvPath = ([&] {
             auto maybeDrvPath = evaluator.store->maybeParseStorePath(arg);
             if (maybeDrvPath && maybeDrvPath->isDerivation()) {