diff --git a/CHANGELOG-unreleased.md b/CHANGELOG-unreleased.md
index 7f5d1743e..be0944adb 100644
--- a/CHANGELOG-unreleased.md
+++ b/CHANGELOG-unreleased.md
@@ -48,4 +48,5 @@ the released changes.
 - Emit warnings when `WaveX`/`DMWaveX` is used together with other representations of red/DM noise
 - `get_observatory()` no longer overwrites `include_gps` and `include_bipm` of `Observatory` objects unless explicitly stated (BIPM and GPS clock corrections no longer incorrectly applied to BAT TOAs).
 - Added back `spacecraft` as an alias for `stl_geo`
+- Fix bug 1729 (missing f-string)
 ### Removed
diff --git a/src/pint/models/timing_model.py b/src/pint/models/timing_model.py
index 06e3e1ead..9eeb16daf 100644
--- a/src/pint/models/timing_model.py
+++ b/src/pint/models/timing_model.py
@@ -27,7 +27,6 @@
 
 """
 
-
 import abc
 import copy
 import inspect
@@ -1915,8 +1914,8 @@ def d_delay_d_param(self, toas, param, acc_delay=None):
         delay_derivs = self.delay_deriv_funcs
         if param not in list(delay_derivs.keys()):
             raise AttributeError(
-                "Derivative function for '{param}' is not provided"
-                " or not registered; parameter '{param}' may not be fittable. "
+                f"Derivative function for '{param}' is not provided"
+                f" or not registered; parameter '{param}' may not be fittable. "
             )
         for df in delay_derivs[param]:
             result += df(toas, param, acc_delay).to(
@@ -2898,7 +2897,7 @@ def __getitem__(self, name):
         for cp in self.components.values():
             if name in cp.params:
                 return getattr(cp, name)
-        raise KeyError("TimingModel does not have parameter {}".format(name))
+        raise KeyError(f"TimingModel does not have parameter {name}")
 
     def __setitem__(self, name, value):
         # FIXME: This could be the right way to add Parameters?
@@ -3121,9 +3120,11 @@ def get_derived_params(self, rms=None, ntoas=None, returndict=False):
                 tasc = ufloat(
                     # This is a time in MJD
                     self.TASC.quantity.mjd,
-                    self.TASC.uncertainty.to(u.d).value
-                    if self.TASC.uncertainty is not None
-                    else 0,
+                    (
+                        self.TASC.uncertainty.to(u.d).value
+                        if self.TASC.uncertainty is not None
+                        else 0
+                    ),
                 )
                 s += "Conversion from ELL1 parameters:\n"
                 ecc = um.sqrt(eps1**2 + eps2**2)