diff --git a/supabase/_async/client.py b/supabase/_async/client.py
index fed0d1ce..f21cd762 100644
--- a/supabase/_async/client.py
+++ b/supabase/_async/client.py
@@ -16,7 +16,7 @@
 from storage3.constants import DEFAULT_TIMEOUT as DEFAULT_STORAGE_CLIENT_TIMEOUT
 from supafunc import AsyncFunctionsClient
 
-from supabase.lib.helpers import is_jwt
+from supabase.lib.helpers import is_valid_jwt
 
 from ..lib.client_options import AsyncClientOptions as ClientOptions
 from .auth_client import AsyncSupabaseAuthClient
@@ -280,7 +280,7 @@ def _create_auth_header(self, token: str):
 
     def _get_auth_headers(self, authorization: Optional[str] = None) -> Dict[str, str]:
         if authorization is None:
-            if is_jwt(self.supabase_key):
+            if is_valid_jwt(self.supabase_key):
                 authorization = self.options.headers.get(
                     "Authorization", self._create_auth_header(self.supabase_key)
                 )
@@ -294,7 +294,9 @@ def _get_auth_headers(self, authorization: Optional[str] = None) -> Dict[str, st
     def _listen_to_auth_events(
         self, event: AuthChangeEvent, session: Optional[Session]
     ):
-        default_access_token = self.supabase_key if is_jwt(self.supabase_key) else None
+        default_access_token = (
+            self.supabase_key if is_valid_jwt(self.supabase_key) else None
+        )
         access_token = default_access_token
         if event in ["SIGNED_IN", "TOKEN_REFRESHED", "SIGNED_OUT"]:
             # reset postgrest and storage instance on event change
diff --git a/supabase/_sync/client.py b/supabase/_sync/client.py
index 4ba21393..c1f9cdd5 100644
--- a/supabase/_sync/client.py
+++ b/supabase/_sync/client.py
@@ -15,7 +15,7 @@
 from storage3.constants import DEFAULT_TIMEOUT as DEFAULT_STORAGE_CLIENT_TIMEOUT
 from supafunc import SyncFunctionsClient
 
-from supabase.lib.helpers import is_jwt
+from supabase.lib.helpers import is_valid_jwt
 
 from ..lib.client_options import SyncClientOptions as ClientOptions
 from .auth_client import SyncSupabaseAuthClient
@@ -279,7 +279,7 @@ def _create_auth_header(self, token: str):
 
     def _get_auth_headers(self, authorization: Optional[str] = None) -> Dict[str, str]:
         if authorization is None:
-            if is_jwt(self.supabase_key):
+            if is_valid_jwt(self.supabase_key):
                 authorization = self.options.headers.get(
                     "Authorization", self._create_auth_header(self.supabase_key)
                 )
@@ -293,7 +293,9 @@ def _get_auth_headers(self, authorization: Optional[str] = None) -> Dict[str, st
     def _listen_to_auth_events(
         self, event: AuthChangeEvent, session: Optional[Session]
     ):
-        default_access_token = self.supabase_key if is_jwt(self.supabase_key) else None
+        default_access_token = (
+            self.supabase_key if is_valid_jwt(self.supabase_key) else None
+        )
         access_token = default_access_token
         if event in ["SIGNED_IN", "TOKEN_REFRESHED", "SIGNED_OUT"]:
             # reset postgrest and storage instance on event change
diff --git a/supabase/lib/helpers.py b/supabase/lib/helpers.py
index 4fb573ed..0b8b308b 100644
--- a/supabase/lib/helpers.py
+++ b/supabase/lib/helpers.py
@@ -1,27 +1,41 @@
 import re
+from typing import Dict
 
 BASE64URL_REGEX = r"^([a-z0-9_-]{4})*($|[a-z0-9_-]{3}$|[a-z0-9_-]{2}$)$"
 
 
-def is_jwt(value: str) -> bool:
-    if value.startswith("Bearer "):
-        value = value.replace("Bearer ", "")
+def is_valid_jwt(value: str) -> bool:
+    """Checks if value looks like a JWT, does not do any extra parsing."""
+    if not isinstance(value, str):
+        return False
 
+    # Remove trailing whitespaces if any.
     value = value.strip()
-    if not value:
-        return False
 
-    parts = value.split(".")
-    if len(parts) != 3:
+    # Remove "Bearer " prefix if any.
+    if value.startswith("Bearer "):
+        value = value[7:]
+
+    # Valid JWT must have 2 dots (Header.Paylod.Signature)
+    if value.count(".") != 2:
         return False
 
-    # loop through the parts and test against regex
-    for part in parts:
-        if len(part) < 4 or not re.search(BASE64URL_REGEX, part, re.IGNORECASE):
+    for part in value.split("."):
+        if not re.search(BASE64URL_REGEX, part, re.IGNORECASE):
             return False
 
     return True
 
 
-def check_authorization_header(headers):
+def check_authorization_header(headers: Dict[str, str]):
+    authorization = headers.get("Authorization")
+    if not authorization:
+        return
+
+    if authorization.startswith("Bearer "):
+        if not is_valid_jwt(authorization):
+            raise ValueError(
+                "create_client called with global Authorization header that does not contain a JWT"
+            )
+
     return True