diff --git a/lix/libutil/args.cc b/lix/libutil/args.cc
index bcd142607..9a75b99ee 100644
--- a/lix/libutil/args.cc
+++ b/lix/libutil/args.cc
@@ -86,7 +86,13 @@ void RootArgs::parseCmdline(const Strings & _cmdline)
     Strings cmdline(_cmdline);
 
     if (auto s = getEnv("NIX_GET_COMPLETIONS")) {
-        size_t n = std::stoi(*s);
+        size_t n = [&] {
+            if (auto parsed = string2Int<size_t>(*s)) {
+                return *parsed;
+            }
+            throw UsageError("Invalid value for environment variable NIX_GET_COMPLETIONS: %s", *s);
+        }();
+
         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;
diff --git a/tests/functional/completions.sh b/tests/functional/completions.sh
index 610c15529..378bd1c15 100644
--- a/tests/functional/completions.sh
+++ b/tests/functional/completions.sh
@@ -37,6 +37,11 @@ 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 value cant be parsed as a number
+NIX_GET_COMPLETIONS="-" expectStderr 1 nix \
+    | grepQuiet "error: Invalid value for environment variable NIX_GET_COMPLETIONS:"
+
 # 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:"