diff --git a/src/gentropy/method/colocalisation.py b/src/gentropy/method/colocalisation.py
index 899343998..3e4e91c74 100644
--- a/src/gentropy/method/colocalisation.py
+++ b/src/gentropy/method/colocalisation.py
@@ -104,8 +104,12 @@ class Coloc:
 
         Coloc requires the availability of Bayes factors (BF) for each variant in the credible set (`logBF` column).
 
+    Attributes:
+        PSEUDOCOUNT (float): Pseudocount to avoid log(0). Defaults to 1e-10.
     """
 
+    PSEUDOCOUNT: float = 1e-10
+
     @staticmethod
     def _get_posteriors(all_bfs: NDArray[np.float64]) -> DenseVector:
         """Calculate posterior probabilities for each hypothesis.
@@ -137,6 +141,7 @@ def colocalise(
 
         Args:
             overlapping_signals (StudyLocusOverlap): overlapping peaks
+
             priorc1 (float): Prior on variant being causal for trait 1. Defaults to 1e-4.
             priorc2 (float): Prior on variant being causal for trait 2. Defaults to 1e-4.
             priorc12 (float): Prior on variant being causal for traits 1 and 2. Defaults to 1e-5.
@@ -188,12 +193,12 @@ def colocalise(
                 .withColumn("lH2bf", f.log(f.col("priorc2")) + f.col("logsum2"))
                 # h3
                 .withColumn("sumlogsum", f.col("logsum1") + f.col("logsum2"))
-                # exclude null H3/H4s: due to sumlogsum == logsum12
-                .filter(f.col("sumlogsum") != f.col("logsum12"))
                 .withColumn("max", f.greatest("sumlogsum", "logsum12"))
                 .withColumn(
                     "logdiff",
-                    (
+                    f.when(
+                        f.col("sumlogsum") == f.col("logsum12"), Coloc.PSEUDOCOUNT
+                    ).otherwise(
                         f.col("max")
                         + f.log(
                             f.exp(f.col("sumlogsum") - f.col("max"))
diff --git a/tests/gentropy/method/test_colocalisation_method.py b/tests/gentropy/method/test_colocalisation_method.py
index a4fe07646..f90d54b3f 100644
--- a/tests/gentropy/method/test_colocalisation_method.py
+++ b/tests/gentropy/method/test_colocalisation_method.py
@@ -41,6 +41,140 @@ def test_coloc_colocalise(
         assert difference.filter(f.abs(f.col(col)) > threshold).count() == 0
 
 
+def test_single_snp_coloc(
+    spark: SparkSession,
+    threshold: float = 1e-5,
+) -> None:
+    """Test edge case of coloc where only one causal SNP is present in the StudyLocusOverlap."""
+    test_overlap_df = spark.createDataFrame(
+        [
+            {
+                "leftStudyLocusId": 1,
+                "rightStudyLocusId": 2,
+                "chromosome": "1",
+                "tagVariantId": "snp",
+                "left_logBF": 10.3,
+                "right_logBF": 10.5,
+            }
+        ]
+    )
+    test_overlap = StudyLocusOverlap(
+        test_overlap_df.select(
+            "leftStudyLocusId",
+            "rightStudyLocusId",
+            "chromosome",
+            "tagVariantId",
+            f.struct(f.col("left_logBF"), f.col("right_logBF")).alias("statistics"),
+        ),
+        StudyLocusOverlap.get_schema(),
+    )
+    test_result = Coloc.colocalise(test_overlap)
+
+    expected = spark.createDataFrame(
+        [
+            {
+                "h0": 9.254841951638903e-5,
+                "h1": 2.7517068829182966e-4,
+                "h2": 3.3609423764447284e-4,
+                "h3": 9.254841952564387e-13,
+                "h4": 0.9992961866536217,
+            }
+        ]
+    )
+    difference = test_result.df.select("h0", "h1", "h2", "h3", "h4").subtract(expected)
+    for col in difference.columns:
+        assert difference.filter(f.abs(f.col(col)) > threshold).count() == 0
+
+
+def test_single_snp_coloc_one_negative(
+    spark: SparkSession,
+    threshold: float = 1e-5,
+) -> None:
+    """Test edge case of coloc where only one causal SNP is present (On one side!) in the StudyLocusOverlap."""
+    test_overlap_df = spark.createDataFrame(
+        [
+            {
+                "leftStudyLocusId": 1,
+                "rightStudyLocusId": 2,
+                "chromosome": "1",
+                "tagVariantId": "snp",
+                "left_logBF": 18.3,
+                "right_logBF": 0.01,
+            }
+        ]
+    )
+    test_overlap = StudyLocusOverlap(
+        test_overlap_df.select(
+            "leftStudyLocusId",
+            "rightStudyLocusId",
+            "chromosome",
+            "tagVariantId",
+            f.struct(f.col("left_logBF"), f.col("right_logBF")).alias("statistics"),
+        ),
+        StudyLocusOverlap.get_schema(),
+    )
+    test_result = Coloc.colocalise(test_overlap)
+    test_result.df.show(1, False)
+    expected = spark.createDataFrame(
+        [
+            {
+                "h0": 1.0246538505087709e-4,
+                "h1": 0.9081680002273896,
+                "h2": 1.0349517929098209e-8,
+                "h3": 1.0246538506112363e-12,
+                "h4": 0.09172952403701702,
+            }
+        ]
+    )
+    difference = test_result.df.select("h0", "h1", "h2", "h3", "h4").subtract(expected)
+    for col in difference.columns:
+        assert difference.filter(f.abs(f.col(col)) > threshold).count() == 0
+
+
+def test_single_snp_coloc_both_negative(
+    spark: SparkSession,
+    threshold: float = 1e-5,
+) -> None:
+    """Test edge case of coloc where only one non-causal SNP overlaps in the StudyLocusOverlap."""
+    test_overlap_df = spark.createDataFrame(
+        [
+            {
+                "leftStudyLocusId": 1,
+                "rightStudyLocusId": 2,
+                "chromosome": "1",
+                "tagVariantId": "snp",
+                "left_logBF": 0.03,
+                "right_logBF": 0.01,
+            }
+        ]
+    )
+    test_overlap = StudyLocusOverlap(
+        test_overlap_df.select(
+            "leftStudyLocusId",
+            "rightStudyLocusId",
+            "chromosome",
+            "tagVariantId",
+            f.struct(f.col("left_logBF"), f.col("right_logBF")).alias("statistics"),
+        ),
+        StudyLocusOverlap.get_schema(),
+    )
+    test_result = Coloc.colocalise(test_overlap)
+    expected = spark.createDataFrame(
+        [
+            {
+                "h0": 0.9997855774090624,
+                "h1": 1.0302335812225042e-4,
+                "h2": 1.0098335895103664e-4,
+                "h3": 9.9978557750904e-9,
+                "h4": 1.0405876008495098e-5,
+            }
+        ]
+    )
+    difference = test_result.df.select("h0", "h1", "h2", "h3", "h4").subtract(expected)
+    for col in difference.columns:
+        assert difference.filter(f.abs(f.col(col)) > threshold).count() == 0
+
+
 def test_ecaviar(mock_study_locus_overlap: StudyLocusOverlap) -> None:
     """Test eCAVIAR."""
     assert isinstance(ECaviar.colocalise(mock_study_locus_overlap), Colocalisation)