diff --git a/rslib/src/deckconfig/mod.rs b/rslib/src/deckconfig/mod.rs
index c522ea18a..e8fb0c0c0 100644
--- a/rslib/src/deckconfig/mod.rs
+++ b/rslib/src/deckconfig/mod.rs
@@ -110,9 +110,9 @@ impl DeckConfig {
 
     /// Retrieve the FSRS 6.0 params, falling back on 5.0 or 4.x ones.
     pub fn fsrs_params(&self) -> &Vec<f32> {
-        if self.inner.fsrs_params_6.len() == 21 {
+        if !self.inner.fsrs_params_6.is_empty() {
             &self.inner.fsrs_params_6
-        } else if self.inner.fsrs_params_5.len() == 19 {
+        } else if !self.inner.fsrs_params_5.is_empty() {
             &self.inner.fsrs_params_5
         } else {
             &self.inner.fsrs_params_4
diff --git a/ts/routes/deck-options/ParamsInput.svelte b/ts/routes/deck-options/ParamsInput.svelte
index 595e02c14..d046c5e84 100644
--- a/ts/routes/deck-options/ParamsInput.svelte
+++ b/ts/routes/deck-options/ParamsInput.svelte
@@ -4,6 +4,7 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
 -->
 <script lang="ts">
     import { tick } from "svelte";
+    import * as tr from "@generated/ftl";
 
     export let value: number[];
     export let defaults: number[];
@@ -28,12 +29,22 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
         return params.map((v) => v.toFixed(4)).join(", ");
     }
 
-    function update(this: HTMLInputElement): void {
-        value = this.value
+    const validParamCounts = [0, 17, 19, 21];
+
+    function update(e: Event): void {
+        const input = e.target as HTMLTextAreaElement;
+        const newValue = input.value
             .replace(/ /g, "")
             .split(",")
             .filter((e) => e)
             .map((v) => Number(v));
+
+        if (validParamCounts.includes(newValue.length)) {
+            value = newValue;
+        } else {
+            alert(tr.deckConfigInvalidParameters());
+            input.value = stringValue;
+        }
     }
 </script>