diff --git a/tangelo/toolboxes/ansatz_generator/hea.py b/tangelo/toolboxes/ansatz_generator/hea.py
index f203369b9..83f94dbe6 100644
--- a/tangelo/toolboxes/ansatz_generator/hea.py
+++ b/tangelo/toolboxes/ansatz_generator/hea.py
@@ -52,8 +52,10 @@ def __init__(self, molecule=None, mapping="jw", up_then_down=False,
                 n_layers=2, rot_type="euler", n_qubits=None, n_electrons=None,
                 reference_state="HF"):
 
-        if not (bool(molecule) ^ (bool(n_qubits) and (bool(n_electrons) | reference_state == "zero"))):
-            raise ValueError(f"A molecule OR qubit + electrons number must be provided when instantiating the HEA.")
+        if not (bool(molecule) ^ (bool(n_qubits) and (bool(n_electrons) | (reference_state == "zero")))):
+            raise ValueError(f"A molecule OR qubit + electrons number must be "
+                "provided when instantiating the HEA with the HF reference state. "
+                "For reference_state='zero', only the number of qubits is needed.")
 
         if n_qubits:
             self.n_qubits = n_qubits