diff --git a/services/github/pulls_manager.py b/services/github/pulls_manager.py
index dcf75d59..1c277fe4 100644
--- a/services/github/pulls_manager.py
+++ b/services/github/pulls_manager.py
@@ -2,6 +2,7 @@
 from config import GITHUB_API_URL, TIMEOUT, PER_PAGE
 from services.github.create_headers import create_headers
 from services.github.github_types import BaseArgs
+from services.github.user_manager import check_user_is_collaborator
 from utils.handle_exceptions import handle_exceptions
 
 
@@ -15,9 +16,24 @@ def add_reviewers(base_args: BaseArgs):
         base_args["token"],
         base_args["reviewers"],
     )
+
+    # Check if the reviewers are collaborators because reviewers must be collaborators
+    valid_reviewers: list[str] = []
+    for reviewer in reviewers:
+        is_collaborator = check_user_is_collaborator(
+            owner=owner, repo=repo, user=reviewer, token=token
+        )
+        if is_collaborator:
+            valid_reviewers.append(reviewer)
+
+    # If no valid reviewers, return
+    if not valid_reviewers:
+        return
+
+    # Add the reviewers to the pull request
     url = f"{GITHUB_API_URL}/repos/{owner}/{repo}/pulls/{pr_number}/requested_reviewers"
     headers = create_headers(token=token)
-    json = {"reviewers": reviewers}
+    json = {"reviewers": valid_reviewers}
     response = requests.post(url=url, headers=headers, json=json, timeout=TIMEOUT)
     response.raise_for_status()
 
diff --git a/services/github/user_manager.py b/services/github/user_manager.py
new file mode 100644
index 00000000..0d2479c7
--- /dev/null
+++ b/services/github/user_manager.py
@@ -0,0 +1,14 @@
+import requests
+from config import GITHUB_API_URL, TIMEOUT
+from services.github.create_headers import create_headers
+from utils.handle_exceptions import handle_exceptions
+
+
+@handle_exceptions(default_return_value=False, raise_on_error=False)
+def check_user_is_collaborator(owner: str, repo: str, user: str, token: str) -> bool:
+    """https://docs.github.com/en/rest/collaborators/collaborators?apiVersion=2022-11-28#check-if-a-user-is-a-repository-collaborator"""
+    url = f"{GITHUB_API_URL}/repos/{owner}/{repo}/collaborators/{user}"
+    headers = create_headers(token=token)
+    response = requests.get(url=url, headers=headers, timeout=TIMEOUT)
+    response.raise_for_status()
+    return response.status_code == 204