diff --git a/swh/graphql/errors/__init__.py b/swh/graphql/errors/__init__.py
--- a/swh/graphql/errors/__init__.py
+++ b/swh/graphql/errors/__init__.py
@@ -3,7 +3,7 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
-from .errors import ObjectNotFoundError
+from .errors import ObjectNotFoundError, PaginationError
 from .handlers import format_error
 
-__all__ = ["ObjectNotFoundError", "format_error"]
+__all__ = ["ObjectNotFoundError", "PaginationError", "format_error"]
diff --git a/swh/graphql/errors/errors.py b/swh/graphql/errors/errors.py
--- a/swh/graphql/errors/errors.py
+++ b/swh/graphql/errors/errors.py
@@ -6,3 +6,13 @@
 
 class ObjectNotFoundError(Exception):
     """ """
+
+
+class PaginationError(Exception):
+    """ """
+
+    msg: str = "Error in pagination input"
+
+    def __init__(self, message, errors=None):
+        # FIXME, log this error
+        super().__init__(f"{self.msg}: {message}")
diff --git a/swh/graphql/resolvers/base_connection.py b/swh/graphql/resolvers/base_connection.py
--- a/swh/graphql/resolvers/base_connection.py
+++ b/swh/graphql/resolvers/base_connection.py
@@ -4,9 +4,11 @@
 # See top-level LICENSE file for more information
 
 from abc import ABC, abstractmethod
+import binascii
 from dataclasses import dataclass
 from typing import Any, Optional, Type
 
+from swh.graphql.errors import PaginationError
 from swh.graphql.utils import utils
 
 from .base_node import BaseNode
@@ -31,6 +33,7 @@
 
     _node_class: Optional[Type[BaseNode]] = None
     _page_size = 50  # default page size
+    _max_page_size = 1000  # maximum value for the first arg
 
     def __init__(self, obj, info, paged_data=None, **kwargs):
         self.obj = obj
@@ -108,18 +111,28 @@
             for (index, node) in enumerate(self.nodes)
         ]
 
-    def _get_after_arg(self):
-        """
-        Return the decoded next page token
-        override to use a specific token
-        """
-        return utils.get_decoded_cursor(self.kwargs.get("after"))
-
-    def _get_first_arg(self):
-        """
-        page_size is set to 50 by default
-        """
-        return self.kwargs.get("first", self._page_size)
+    def _get_after_arg(self) -> str:
+        """
+        Return the decoded next page token. Override to support a different
+        cursor type
+        """
+        # different cursor is used in SnapshotBranchConnection
+        try:
+            cursor = utils.get_decoded_cursor(self.kwargs.get("after"))
+        except (UnicodeDecodeError, binascii.Error, Exception) as e:
+            raise PaginationError("Invalid value for argument 'after'", errors=e)
+        return cursor
+
+    def _get_first_arg(self) -> int:
+        """ """
+        # page_size is set to 50 by default
+        # Input type check is not required; It is defined in schema as an int
+        first = self.kwargs.get("first", self._page_size)
+        if first < 0 or first > self._max_page_size:
+            raise PaginationError(
+                "Value for argument 'first' is either too big or invalid"
+            )
+        return first
 
     def _get_index_cursor(self, index: int, node: Any):
         """
diff --git a/swh/graphql/resolvers/snapshot_branch.py b/swh/graphql/resolvers/snapshot_branch.py
--- a/swh/graphql/resolvers/snapshot_branch.py
+++ b/swh/graphql/resolvers/snapshot_branch.py
@@ -3,9 +3,11 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+import binascii
 from collections import namedtuple
 
 from swh.graphql.backends import archive
+from swh.graphql.errors import PaginationError
 from swh.graphql.utils import utils
 from swh.storage.interface import PagedResult
 
@@ -54,7 +56,7 @@
         # self.obj.swhid is the snapshot SWHID
         result = archive.Archive().get_snapshot_branches(
             self.obj.swhid.object_id,
-            after=self._get_after_arg(),
+            after=self._get_after_arg().encode(),
             first=self._get_first_arg(),
             target_types=self.kwargs.get("types"),
             name_include=self._get_name_include_arg(),
@@ -71,10 +73,13 @@
             results=result["branches"].items(), next_page_token=end_cusrsor
         )
 
-    def _get_after_arg(self):
+    def _get_after_arg(self) -> str:
         # Snapshot branch is using a different cursor; logic to handle that
-        after = utils.get_decoded_cursor(self.kwargs.get("after", ""))
-        return after.encode() if after else b""
+        try:
+            cursor = utils.get_decoded_cursor(self.kwargs.get("after", ""))
+        except (UnicodeDecodeError, binascii.Error, Exception) as e:
+            raise PaginationError("Invalid value for argument 'after'", errors=e)
+        return cursor
 
     def _get_name_include_arg(self):
         name_include = self.kwargs.get("nameInclude", None)
@@ -82,4 +87,4 @@
 
     def _get_index_cursor(self, index: int, node: SnapshotBranchNode):
         # Snapshot branch is using a different cursor, hence the override
-        return utils.get_encoded_cursor(node.name.hex())
+        return utils.get_encoded_cursor(node.name)
diff --git a/swh/graphql/tests/functional/test_pagination.py b/swh/graphql/tests/functional/test_pagination.py
--- a/swh/graphql/tests/functional/test_pagination.py
+++ b/swh/graphql/tests/functional/test_pagination.py
@@ -8,32 +8,10 @@
 
 
 # Using Origin object to run functional tests for pagination
-def test_pagination(client):
-    # requesting the max number of nodes available
-    # endCursor must be None
-    query_str = f"""
-    {{
-      origins(first: {len(get_origins())}) {{
-        nodes {{
-          id
-        }}
-        pageInfo {{
-          hasNextPage
-          endCursor
-        }}
-      }}
-    }}
-    """
-
-    data, _ = get_query_response(client, query_str)
-    assert len(data["origins"]["nodes"]) == len(get_origins())
-    assert data["origins"]["pageInfo"] == {"hasNextPage": False, "endCursor": None}
-
-
-def get_first_node(client):
+def get_origin_nodes(client, first=1, after=""):
     query_str = """
     {
-      origins(first: 1) {
+      origins(first: %s, %s) {
         nodes {
           id
         }
@@ -43,58 +21,89 @@
         }
       }
     }
-    """
-    data, _ = get_query_response(client, query_str)
-    return data["origins"]
+    """ % (
+        first,
+        after,
+    )
+    return get_query_response(client, query_str)
+
+
+def test_pagination(client):
+    # requesting the max number of nodes available
+    # endCursor must be None
+    data, _ = get_origin_nodes(client, len(get_origins()))
+    assert len(data["origins"]["nodes"]) == len(get_origins())
+    assert data["origins"]["pageInfo"] == {"hasNextPage": False, "endCursor": None}
 
 
 def test_first_arg(client):
-    origins = get_first_node(client)
-    assert len(origins["nodes"]) == 1
-    assert origins["pageInfo"]["hasNextPage"] is True
+    data, _ = get_origin_nodes(client, 1)
+    assert len(data["origins"]["nodes"]) == 1
+    assert data["origins"]["pageInfo"]["hasNextPage"] is True
+
+
+def test_invalid_first_arg(client):
+    data, errors = get_origin_nodes(client, -1)
+    assert data["origins"] is None
+    assert (len(errors)) == 2  # one error for origins and anotehr one for pageInfo
+    assert (
+        errors[0]["message"]
+        == "Error in pagination input: Value for argument 'first' is either too big or invalid"
+    )
+
+
+def test_too_big_first_arg(client):
+    data, errors = get_origin_nodes(client, 1001)  # max page size is 1000
+    assert data["origins"] is None
+    assert (len(errors)) == 2
+    assert (
+        errors[0]["message"]
+        == "Error in pagination input: Value for argument 'first' is either too big or invalid"
+    )
 
 
 def test_after_arg(client):
-    origins = get_first_node(client)
-    end_cursor = origins["pageInfo"]["endCursor"]
-    query_str = f"""
-    {{
-      origins(first: 1, after: "{end_cursor}") {{
-        nodes {{
-          id
-        }}
-        pageInfo {{
-          hasNextPage
-          endCursor
-        }}
-      }}
-    }}
-    """
-    data, _ = get_query_response(client, query_str)
+    first_data, _ = get_origin_nodes(client)
+    end_cursor = first_data["origins"]["pageInfo"]["endCursor"]
+    # get again with endcursor as the after argument
+    data, _ = get_origin_nodes(client, 1, f'after: "{end_cursor}"')
     assert len(data["origins"]["nodes"]) == 1
     assert data["origins"]["pageInfo"] == {"hasNextPage": False, "endCursor": None}
 
 
+def test_invalid_after_arg(client):
+    data, errors = get_origin_nodes(client, 1, 'after: "invalid"')
+    assert data["origins"] is None
+    assert (len(errors)) == 2
+    assert (
+        errors[0]["message"]
+        == "Error in pagination input: Invalid value for argument 'after'"
+    )
+
+
 def test_edge_cursor(client):
-    origins = get_first_node(client)
+    origins = get_origin_nodes(client)[0]["origins"]
     # end cursor here must be the item cursor for the second item
     end_cursor = origins["pageInfo"]["endCursor"]
 
-    query_str = f"""
-    {{
-      origins(first: 1, after: "{end_cursor}") {{
-        edges {{
+    query_str = (
+        """
+    {
+      origins(first: 1, after: "%s") {
+        edges {
           cursor
-          node {{
+          node {
             id
-          }}
-        }}
-        nodes {{
+          }
+        }
+        nodes {
           id
-        }}
-      }}
-    }}
+        }
+      }
+    }
     """
+        % end_cursor
+    )
     data, _ = get_query_response(client, query_str)
     origins = data["origins"]
     assert [edge["node"] for edge in origins["edges"]] == origins["nodes"]
diff --git a/swh/graphql/utils/utils.py b/swh/graphql/utils/utils.py
--- a/swh/graphql/utils/utils.py
+++ b/swh/graphql/utils/utils.py
@@ -27,7 +27,7 @@
 def get_decoded_cursor(cursor: str) -> str:
     if cursor is None:
         return None
-    return base64.b64decode(cursor).decode(ENCODING)
+    return base64.b64decode(cursor, validate=True).decode()
 
 
 def str_to_sha1(sha1: str) -> bytearray: