diff --git a/src/qrules/particle.py b/src/qrules/particle.py
index 81085ff8..9ddc931d 100644
--- a/src/qrules/particle.py
+++ b/src/qrules/particle.py
@@ -180,7 +180,7 @@ def __attrs_post_init__(self) -> None:
                 f" Q[{self.charge}] !="
                 f" Iz[{self.isospin.projection if self.isospin else 0}] + 1/2"
                 f" (B[{self.baryon_number}] +  S[{self.strangeness}] + "
-                f" C[{self.charmness}] + B'[{self.bottomness}] + T[{self.strangeness}])"
+                f" C[{self.charmness}] + B'[{self.bottomness}] + T[{self.topness}])"
             )
             raise ValueError(msg)