diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 144f8f886..6d5ee61be 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -706,9 +706,15 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
 
         auto [cursor, attrPath] = installable.getCursor(*evalState);
 
-        auto templateDir = cursor->getAttr("path")->getString();
+        auto templateDirAttr = cursor->getAttr("path");
+        auto templateDir = templateDirAttr->getString();
 
-        assert(store->isInStore(templateDir));
+        if (!store->isInStore(templateDir)) {
+            throw TypeError(
+                std::string("'%s' was not found in the Nix store\n") +
+                "If you've set '%s' to a string, try using a path instead.",
+                templateDir, templateDirAttr->getAttrPathStr());
+        }
 
         std::vector<Path> files;