diff --git a/lib/charms/prometheus_k8s/v0/prometheus_remote_write.py b/lib/charms/prometheus_k8s/v0/prometheus_remote_write.py
index 07a379f3..910d7ca5 100644
--- a/lib/charms/prometheus_k8s/v0/prometheus_remote_write.py
+++ b/lib/charms/prometheus_k8s/v0/prometheus_remote_write.py
@@ -45,7 +45,7 @@
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 10
+LIBPATCH = 11
 
 
 logger = logging.getLogger(__name__)
@@ -58,7 +58,11 @@
 DEFAULT_ALERT_RULES_RELATIVE_PATH = "./src/prometheus_alert_rules"
 
 
-class RelationNotFoundError(Exception):
+class RemoteWriteError(Exception):
+    """Base class remote-write errors."""
+
+
+class RelationNotFoundError(RemoteWriteError):
     """Raised if there is no relation with the given name."""
 
     def __init__(self, relation_name: str):
@@ -68,7 +72,7 @@ def __init__(self, relation_name: str):
         super().__init__(self.message)
 
 
-class RelationInterfaceMismatchError(Exception):
+class RelationInterfaceMismatchError(RemoteWriteError):
     """Raised if the relation with the given name has a different interface."""
 
     def __init__(
@@ -89,7 +93,7 @@ def __init__(
         super().__init__(self.message)
 
 
-class RelationRoleMismatchError(Exception):
+class RelationRoleMismatchError(RemoteWriteError):
     """Raised if the relation with the given name has a different direction."""
 
     def __init__(
@@ -445,7 +449,7 @@ def restore(self, snapshot):
         self.relation_id = snapshot["relation_id"]
 
 
-class InvalidAlertRulePathError(Exception):
+class InvalidAlertRulePathError(RemoteWriteError):
     """Raised if the alert rules folder cannot be found or is otherwise invalid."""
 
     def __init__(
@@ -764,7 +768,7 @@ def __init__(
         self,
         charm: CharmBase,
         relation_name: str = DEFAULT_RELATION_NAME,
-        endpoint_schema: str = "http",
+        endpoint_schema: str = "http",  # TODO: in v1, rename to 'scheme'
         endpoint_address: str = "",
         endpoint_port: Union[str, int] = 9090,
         endpoint_path: str = "/api/v1/write",
@@ -802,7 +806,7 @@ def __init__(
         self._charm = charm
         self.tool = CosTool(self._charm)
         self._relation_name = relation_name
-        self._endpoint_schema = endpoint_schema
+        self._endpoint_scheme = endpoint_schema.strip().rstrip("://")
         self._endpoint_address = endpoint_address
         self._endpoint_port = int(endpoint_port)
         self._endpoint_path = endpoint_path
@@ -851,8 +855,12 @@ def _set_endpoint_on_relation(self, relation: Relation) -> None:
         if path and not path.startswith("/"):
             path = "/{}".format(path)
 
+        if not re.match(r"^\w+$", self._endpoint_scheme):
+            msg = f"Invalid scheme 'f{self._endpoint_scheme}'; did you mean 'http'?"
+            raise ValueError(msg)
+
         endpoint_url = "{}://{}:{}{}".format(
-            self._endpoint_schema, address, str(self._endpoint_port), path
+            self._endpoint_scheme, address, str(self._endpoint_port), path
         )
 
         relation.data[self._charm.unit]["remote_write"] = json.dumps(
diff --git a/tox.ini b/tox.ini
index fd4e7c25..92825804 100644
--- a/tox.ini
+++ b/tox.ini
@@ -73,7 +73,7 @@ deps =
     integration: pytest-operator==1.0.0b1
 commands =
     charm: mypy {[vars]src_path} {posargs}
-    lib: mypy --python-version 3.5 {[vars]lib_path} {posargs}
+    lib: mypy --python-version 3.8 {[vars]lib_path} {posargs}
     unit: mypy {[vars]tst_path}/unit {posargs}
     integration: mypy {[vars]tst_path}/integration {posargs}