diff --git a/finemapper.py b/finemapper.py
index 9d450b9..8f4dc70 100644
--- a/finemapper.py
+++ b/finemapper.py
@@ -782,6 +782,8 @@ def finemap(self, locus_start, locus_end, num_causal_snps, use_prior_causal_prob
             prior_var = h2_hess / num_causal_snps
             if prior_var <= 0:
                 raise ValueError('HESS estimates that the locus causally explains zero heritability')
+            if prior_var >= 1:
+                raise ValueError('HESS-estimated prior-var >1. The HESS estimator cannot be used in this locus.')
             logging.info('HESS estimated causal effect size variance: %0.4e'%(prior_var))
             
             if hess_resvar: