diff --git a/pyproject.toml b/pyproject.toml
index 055d0a1..dedf28a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "scgenerator"
-version = "0.3.15"
+version = "0.3.16"
 description = "Simulate nonlinear pulse propagation in optical fibers"
 readme = "README.md"
 authors = [{ name = "Benoit Sierro", email = "benoit.sierro@iap.unibe.ch" }]
diff --git a/src/scgenerator/spectra.py b/src/scgenerator/spectra.py
index df8f417..88570ae 100644
--- a/src/scgenerator/spectra.py
+++ b/src/scgenerator/spectra.py
@@ -253,7 +253,7 @@ class Propagation(Generic[ParamsOrNone]):
     def _warn_negative_index(self, index: int | None):
         if (index is not None and index >= 0) or self.parameters is None:
             return
-        if self._current_index < len(self.z_positions):
+        if self._current_index < self.parameters.z_num:
             warnings.warn(f"attempting to access index {index} on an incomplete propagation obj")
 
     def load_all(self) -> Spectrum: