diff --git a/src/NixStore.zig b/src/NixStore.zig
index 0fd66b9..b7db990 100644
--- a/src/NixStore.zig
+++ b/src/NixStore.zig
@@ -14,6 +14,8 @@ pub fn add(path: []const u8) error{Failure}![]const u8 {
         .argv = &[_][]const u8{ "nix", "store", "add", path },
     }) catch return error.Failure;
 
+    if (res.term != .Exited or res.term.Exited != 0) return error.Failure;
+    if (res.stdout.len == 0) return error.Failure;
     return res.stdout[0 .. res.stdout.len - 1]; // to chop off the \n
 }