diff --git a/frontend/packages/swarm-ui/src/pages/review-tryout/ReviewTryOutPage.tsx b/frontend/packages/swarm-ui/src/pages/review-tryout/ReviewTryOutPage.tsx
index 25c3a763..78fcc663 100644
--- a/frontend/packages/swarm-ui/src/pages/review-tryout/ReviewTryOutPage.tsx
+++ b/frontend/packages/swarm-ui/src/pages/review-tryout/ReviewTryOutPage.tsx
@@ -41,6 +41,7 @@ import {
   prNumber,
   repoProblem,
   rowsProblem,
+  runProblem,
   toBody,
   type ClassificationDraft,
   type OpenPrAnswer,
@@ -218,9 +219,7 @@ export function ReviewTryOutPage() {
   }, [runId]);
 
   const openPrProblem = rowsProblem(repo, drafts);
-  const problem =
-    (prNumber(pr) === null ? "a PR number is a whole number above 0" : null) ??
-    openPrProblem;
+  const problem = runProblem(repo, pr, drafts);
 
   async function run(e: Event) {
     e.preventDefault();
diff --git a/frontend/packages/swarm-ui/src/pages/review-tryout/TryOutTypes.ts b/frontend/packages/swarm-ui/src/pages/review-tryout/TryOutTypes.ts
index 0195ac1f..7143ac3c 100644
--- a/frontend/packages/swarm-ui/src/pages/review-tryout/TryOutTypes.ts
+++ b/frontend/packages/swarm-ui/src/pages/review-tryout/TryOutTypes.ts
@@ -164,6 +164,21 @@ export function rowsProblem(
   );
 }
 
+/** What keeps the rows from a run, or null: everything `rowsProblem` checks,
+ * then the PR number. The page shows this one reason for both of its buttons,
+ * so whatever disables opening a PR comes before the PR number it doesn't
+ * need. */
+export function runProblem(
+  repo: string,
+  pr: string,
+  drafts: ClassificationDraft[],
+): string | null {
+  return (
+    rowsProblem(repo, drafts) ??
+    (prNumber(pr) === null ? "a PR number is a whole number above 0" : null)
+  );
+}
+
 export function toBody(d: ClassificationDraft): ClassificationBody {
   return {
     name: d.name.trim(),
diff --git a/frontend/packages/swarm-ui/test/tryout-types.test.mjs b/frontend/packages/swarm-ui/test/tryout-types.test.mjs
index 95fa18af..c64d036f 100644
--- a/frontend/packages/swarm-ui/test/tryout-types.test.mjs
+++ b/frontend/packages/swarm-ui/test/tryout-types.test.mjs
@@ -12,6 +12,7 @@ import {
   prNumber,
   repoProblem,
   rowsProblem,
+  runProblem,
   thresholdValue,
   toBody,
 } from "../src/pages/review-tryout/TryOutTypes.ts";
@@ -130,3 +131,13 @@ test("only a typed name or description counts as the user's work", () => {
   assert.equal(hasTypedText([row({ name: " ", description: "" })]), false);
   assert.equal(hasTypedText([row({ name: "", description: "x" })]), true);
 });
+
+test("the page's one hint names what disables opening a PR before the PR number", () => {
+  // No PR number and a bad row: the row is what disables both buttons.
+  assert.match(runProblem("o/r", "", [row({ name: "" })]), /a name/);
+  assert.match(runProblem("o", "", [row()]), /owner\/name/);
+  // Rows ready but no PR number: only the run is held back.
+  assert.match(runProblem("o/r", "", [row()]), /PR number/);
+  assert.equal(rowsProblem("o/r", [row()]), null);
+  assert.equal(runProblem("o/r", "7", [row()]), null);
+});