diff --git a/src/quantumhall_matrixelements/exchange_legendre.py b/src/quantumhall_matrixelements/exchange_legendre.py
index 0bbf0cb..638c4f1 100644
--- a/src/quantumhall_matrixelements/exchange_legendre.py
+++ b/src/quantumhall_matrixelements/exchange_legendre.py
@@ -65,6 +65,9 @@ def get_exchange_kernels_GaussLegendre(
     # -----------------------------
     # 0. Input handling
     # -----------------------------
+    if sign_magneticfield not in (1, -1):
+        raise ValueError("sign_magneticfield must be 1 or -1")
+
     G_magnitudes = np.asarray(G_magnitudes, dtype=float)
     G_angles = np.asarray(G_angles, dtype=float)
     if G_magnitudes.shape != G_angles.shape: