diff --git a/euphonic/spectra.py b/euphonic/spectra.py
index 476ef33d9..a807c786a 100644
--- a/euphonic/spectra.py
+++ b/euphonic/spectra.py
@@ -1981,7 +1981,7 @@ def _from_spectra_bins_check(ref_bins_data: dict[str, Quantity],
 
     @classmethod
     def from_spectra(
-            cls, spectra: Sequence[Spectrum2D], unsafe: bool = False
+            cls, spectra: Sequence[Spectrum2D], *, unsafe: bool = False
     ) -> Self:
         """Combine Spectrum2D to produce a new collection