diff --git a/cozo-core/src/runtime/imperative.rs b/cozo-core/src/runtime/imperative.rs
index 751dadcc..9b62ca67 100644
--- a/cozo-core/src/runtime/imperative.rs
+++ b/cozo-core/src/runtime/imperative.rs
@@ -260,12 +260,12 @@ impl<'s, S: Storage<'s>> Db<S> {
     ) -> Result<NamedRows, Report> {
         let mut callback_collector = BTreeMap::new();
         let mut write_lock_names = BTreeSet::new();
-        if readonly && !write_lock_names.is_empty() {
-            bail!("Read-only imperative program attempted to acquire write locks");
-        }
         for p in ps {
             p.needs_write_locks(&mut write_lock_names);
         }
+        if readonly && !write_lock_names.is_empty() {
+            bail!("Read-only imperative program attempted to acquire write locks");
+        }
         let is_write = !write_lock_names.is_empty();
         let write_lock = self.obtain_relation_locks(write_lock_names.iter());
         let _write_lock_guards = write_lock.iter().map(|l| l.read().unwrap()).collect_vec();