diff --git a/crates/engine/src/cmd/data.rs b/crates/engine/src/cmd/data.rs
index a33c39166..102115f00 100644
--- a/crates/engine/src/cmd/data.rs
+++ b/crates/engine/src/cmd/data.rs
@@ -955,6 +955,9 @@ fn validation(s: &mut Session, p: &Json) -> Result<Json> {
         // An unknown type used to give an "any value" rule, saved as `type="none"`: no validation.
         other => return Err(bad("data.validation", format!("unknown type `{other}` (any, whole, decimal, list, date, time, textLength, custom)"))),
     };
+    if kind == ValidationKind::List && str_param(p, "formula1").is_none_or(|f| f.trim().trim_start_matches('=').is_empty()) {
+        return Err(bad("data.validation", "a list validation needs `formula1`"));
+    }
     const OPERATORS: [&str; 12] = [
         "between",
         "notBetween",
@@ -1402,6 +1405,17 @@ mod tests {
         }
     }
 
+    #[test]
+    fn list_validation_without_formula1_is_an_error() {
+        let mut s = Session::new();
+        s.new_workbook();
+        assert!(s.execute("data.validation", json!({"range": "A1:A3", "type": "list", "source": "\"a,b,c\""})).is_err());
+        assert!(s.execute("data.validation", json!({"range": "A1:A3", "type": "list"})).is_err());
+        assert!(s.doc().unwrap().wb.active().unwrap().validations.is_empty());
+        s.execute("data.validation", json!({"range": "A1:A3", "type": "list", "formula1": "\"a,b,c\""})).unwrap();
+        assert_eq!(s.execute("data.validate", json!({"cell": "A1", "input": "a"})).unwrap()["ok"], json!(true));
+    }
+
     #[test]
     fn partial_validation_edits_keep_the_rest_of_the_old_rule() {
         let mut s = Session::new();