diff --git a/altamisa/isatab/parse_assay_study.py b/altamisa/isatab/parse_assay_study.py
index 0a9c446..a49ac97 100644
--- a/altamisa/isatab/parse_assay_study.py
+++ b/altamisa/isatab/parse_assay_study.py
@@ -769,8 +769,7 @@ def _read_next_line(self):
         except StopIteration:
             self._line = None
         except UnicodeDecodeError as e:  # pragma: no cover
-            tpl = "Invalid encoding after line {} of study file '{}' (use Unicode/UTF-8)."
-            msg = tpl.format(self._reader.line_num, self.filename)
+            msg = f"Invalid encoding of study file '{self._filename}' (use Unicode/UTF-8)."
             raise ParseIsatabException(msg) from e
         return prev_line
 
@@ -893,8 +892,7 @@ def _read_next_line(self):
         except StopIteration:
             self._line = None
         except UnicodeDecodeError as e:  # pragma: no cover
-            tpl = "Invalid encoding after line {} of assay file '{}' (use Unicode/UTF-8)."
-            msg = tpl.format(self._reader.line_num, self.filename)
+            msg = f"Invalid encoding of assay file '{self._filename}' (use Unicode/UTF-8)."
             raise ParseIsatabException(msg) from e
         return prev_line
 
diff --git a/altamisa/isatab/parse_investigation.py b/altamisa/isatab/parse_investigation.py
index 2db728e..a365cbf 100644
--- a/altamisa/isatab/parse_investigation.py
+++ b/altamisa/isatab/parse_investigation.py
@@ -133,8 +133,7 @@ def _read_next_line(self):
         except StopIteration:
             self._line = None
         except UnicodeDecodeError as e:  # pragma: no cover
-            tpl = "Invalid encoding after line {} of investigation file '{}' (use Unicode/UTF-8)."
-            msg = tpl.format(self._reader.line_num, self._filename)
+            msg = f"Invalid encoding of investigation file '{self._filename}' (use Unicode/UTF-8)."
             raise ParseIsatabException(msg) from e
         return prev_line