diff --git a/pythonosc/osc_bundle_builder.py b/pythonosc/osc_bundle_builder.py
index bf42664..ee28d6f 100644
--- a/pythonosc/osc_bundle_builder.py
+++ b/pythonosc/osc_bundle_builder.py
@@ -53,8 +53,7 @@ def build(self) -> osc_bundle.OscBundle:
                     dgram += content.dgram
                 else:
                     raise BuildError(
-                        "Content must be either OscBundle or OscMessage"
-                        "found {}".format(type(content))
+                        f"Content must be either OscBundle or OscMessage, found {type(content)}"
                     )
             return osc_bundle.OscBundle(dgram)
         except osc_types.BuildError as be:
diff --git a/pythonosc/osc_message_builder.py b/pythonosc/osc_message_builder.py
index fc4a035..ebbc2cb 100644
--- a/pythonosc/osc_message_builder.py
+++ b/pythonosc/osc_message_builder.py
@@ -90,9 +90,7 @@ def add_arg(self, arg_value: ArgValue, arg_type: Optional[str] = None) -> None:
         """
         if arg_type and not self._valid_type(arg_type):
             raise ValueError(
-                "arg_type must be one of {}, or an array of valid types".format(
-                    self._SUPPORTED_ARG_TYPES
-                )
+                f"arg_type must be one of {self._SUPPORTED_ARG_TYPES}, or an array of valid types"
             )
         if not arg_type:
             arg_type = self._get_arg_type(arg_value)