diff --git a/.github/workflows/assign_reviewers.yml b/.github/workflows/assign_reviewers.yml
index 5e1ba21110d..578ed30744b 100644
--- a/.github/workflows/assign_reviewers.yml
+++ b/.github/workflows/assign_reviewers.yml
@@ -15,6 +15,7 @@ on:
 jobs:
   requested-reviewer:
     runs-on: ubuntu-latest
+    if: ${{ github.event.requested_reviewer != null }}
     permissions:
       pull-requests: write
     steps:
@@ -26,18 +27,14 @@ jobs:
               if (context.payload.pull_request === undefined) {
               throw new Error("Can't get pull_request payload. " +
                               'Check a request reviewer event was triggered.');
-              }
-              const reviewers = context.payload.pull_request.requested_reviewers;
-              // Assignees takes in a list of logins rather than the
-              // reviewer object.
-              const reviewerNames = reviewers.map(reviewer => reviewer.login);
-              const {number:issue_number} = context.payload.pull_request;
-              github.rest.issues.addAssignees({
-                owner: context.repo.owner,
-                repo: context.repo.repo,
-                issue_number: issue_number,
-                assignees: reviewerNames
-              });
-            } catch (error) {
-              core.setFailed(error.message);
             }
+            const {number:issue_number} = context.payload.pull_request;
+            await github.rest.issues.addAssignees({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: issue_number,
+              assignees: [context.payload.requested_reviewer.login]
+            });
+          } catch (error) {
+            core.setFailed(error.message);
+          }
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