diff --git a/dist/index.js b/dist/index.js
index 8e040f7..081b94d 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -1625,9 +1625,14 @@ function run() {
         }
         else if (eventName === 'workflow_run') {
             if (pullRequestNumberInput === 'not set') {
-                throw Error(`If action is triggered by "workflow_run" then input "pullRequestNumber" is required.`);
+                core.warning(`If action is triggered by "workflow_run" then input "pullRequestNumber" is required.\n` +
+                    `It might be missing because the pull request might have been already merged or a fixup pushed to` +
+                    `the PR branch. None of the outputs will be set as we cannot find the right PR.`);
+                return;
+            }
+            else {
+                pullRequestNumber = parseInt(pullRequestNumberInput);
             }
-            pullRequestNumber = parseInt(pullRequestNumberInput);
         }
         else {
             throw Error(`This action is only useful in "pull_request_review" or "workflow_run" triggered runs and you used it in "${eventName}"`);
diff --git a/src/main.ts b/src/main.ts
index f7b61d6..d9d012c 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -211,11 +211,15 @@ async function run(): Promise<void> {
     }
   } else if (eventName === 'workflow_run') {
     if (pullRequestNumberInput === 'not set') {
-      throw Error(
-        `If action is triggered by "workflow_run" then input "pullRequestNumber" is required.`
+      core.warning(
+        `If action is triggered by "workflow_run" then input "pullRequestNumber" is required.\n` +
+          `It might be missing because the pull request might have been already merged or a fixup pushed to` +
+          `the PR branch. None of the outputs will be set as we cannot find the right PR.`
       )
+      return
+    } else {
+      pullRequestNumber = parseInt(pullRequestNumberInput)
     }
-    pullRequestNumber = parseInt(pullRequestNumberInput)
   } else {
     throw Error(
       `This action is only useful in "pull_request_review" or "workflow_run" triggered runs and you used it in "${eventName}"`