diff --git a/lix/libutil/args.cc b/lix/libutil/args.cc
index d8a88f003..bcd142607 100644
--- a/lix/libutil/args.cc
+++ b/lix/libutil/args.cc
@@ -87,7 +87,8 @@ void RootArgs::parseCmdline(const Strings & _cmdline)
 
     if (auto s = getEnv("NIX_GET_COMPLETIONS")) {
         size_t n = std::stoi(*s);
-        assert(n > 0 && n <= cmdline.size());
+        if (!(n > 0 && n <= cmdline.size()))
+            throw UsageError("Invalid word number to get completion for: %zu\n. Your autocompletions might be misconfigured", n);
         *std::next(cmdline.begin(), n - 1) += completionMarker;
         completions = std::make_shared<Completions>();
         verbosity = lvlError;
diff --git a/tests/functional/completions.sh b/tests/functional/completions.sh
index d3d5bbd48..610c15529 100644
--- a/tests/functional/completions.sh
+++ b/tests/functional/completions.sh
@@ -37,6 +37,12 @@ EOF
 [[ "$(NIX_GET_COMPLETIONS=1 nix buil)" == $'normal\nbuild\t' ]]
 [[ "$(NIX_GET_COMPLETIONS=2 nix flake metad)" == $'normal\nmetadata\t' ]]
 
+# Test how completion  fails if the number is not a valid index for the number of arguments
+NIX_GET_COMPLETIONS=0 expectStderr 1 nix \
+    | grepQuiet "error: Invalid word number to get completion for:"
+NIX_GET_COMPLETIONS=4 expectStderr 1  nix build a \
+    | grepQuiet "error: Invalid word number to get completion for:"
+
 # Filename completion
 [[ "$(NIX_GET_COMPLETIONS=2 nix build ./f)" == $'filenames\n./foo\t' ]]
 [[ "$(NIX_GET_COMPLETIONS=2 nix build ./nonexistent)" == $'filenames' ]]