diff --git a/pyerrors/input/openQCD.py b/pyerrors/input/openQCD.py
index 7cb1a612..302b77df 100644
--- a/pyerrors/input/openQCD.py
+++ b/pyerrors/input/openQCD.py
@@ -569,6 +569,8 @@ def read_qtop(path, prefix, c, dtr_cnfg=1, version="openQCD", **kwargs):
 def read_gf_coupling(path, prefix, c, dtr_cnfg=1, Zeuthen_flow=True, **kwargs):
     """Read the gradient flow coupling based on sfqcd gradient flow measurements. See 1607.06423 for details.
 
+    Note: The current implementation only works for c=0.3 and T=L. The definition of the coupling in 1607.06423 requires projection to topological charge zero which is not done within this function but has to be performed in a separate step.
+
     Parameters
     ----------
     path : str
@@ -600,6 +602,9 @@ def read_gf_coupling(path, prefix, c, dtr_cnfg=1, Zeuthen_flow=True, **kwargs):
         (optional) If True, the Zeuthen flow is used for the coupling. If False, the Wilson flow is used.
     """
 
+    if c != 0.3:
+        raise Exception("The required lattice norm is only implemented for c=0.3 at the moment.")
+
     plaq = _read_flow_obs(path, prefix, c, dtr_cnfg=dtr_cnfg, version="sfqcd", obspos=6, sum_t=False, Zeuthen_flow=Zeuthen_flow, integer_charge=False, **kwargs)
     C2x1 = _read_flow_obs(path, prefix, c, dtr_cnfg=dtr_cnfg, version="sfqcd", obspos=7, sum_t=False, Zeuthen_flow=Zeuthen_flow, integer_charge=False, **kwargs)
     L = int(plaq.tag[2:])
diff --git a/tests/openQCD_in_test.py b/tests/openQCD_in_test.py
index a03c0590..71b47ba2 100644
--- a/tests/openQCD_in_test.py
+++ b/tests/openQCD_in_test.py
@@ -104,3 +104,5 @@ def test_gf_coupling():
     path = './tests//data/openqcd_test/'
     prefix = 'sfqcd'
     gf = pe.input.openQCD.read_gf_coupling(path, prefix, c=0.3)
+    with pytest.raises(Exception):
+        pe.input.openQCD.read_gf_coupling(path, prefix, c=0.35)