diff --git a/src/enlyze/api_clients/timeseries/models.py b/src/enlyze/api_clients/timeseries/models.py
index 84996be..59531db 100644
--- a/src/enlyze/api_clients/timeseries/models.py
+++ b/src/enlyze/api_clients/timeseries/models.py
@@ -84,7 +84,7 @@ def merge(self, other: "TimeseriesData") -> "TimeseriesData":
                 f" with {slen} records."
             )
 
-        self.columns.extend(other.columns)
+        self.columns.extend(other.columns[1:])
 
         for s, o in zip(self.records, other.records):
             if s[0] != o[0]:
diff --git a/tests/enlyze/api_clients/timeseries/test_models.py b/tests/enlyze/api_clients/timeseries/test_models.py
new file mode 100644
index 0000000..59ddd26
--- /dev/null
+++ b/tests/enlyze/api_clients/timeseries/test_models.py
@@ -0,0 +1,76 @@
+from datetime import datetime, timedelta, timezone
+
+import pytest
+
+from enlyze.api_clients.timeseries.models import TimeseriesData
+
+
+@pytest.fixture
+def timestamp():
+    return datetime.now(tz=timezone.utc)
+
+
+@pytest.fixture
+def timeseries_data_1(timestamp):
+    return TimeseriesData(
+        columns=["time", "var1", "var2"],
+        records=[
+            [timestamp.isoformat(), 1, 2],
+            [(timestamp - timedelta(minutes=10)).isoformat(), 3, 4],
+        ],
+    )
+
+
+@pytest.fixture
+def timeseries_data_2(timestamp):
+    return TimeseriesData(
+        columns=["time", "var3"],
+        records=[
+            [timestamp.isoformat(), 5],
+            [(timestamp - timedelta(minutes=10)).isoformat(), 6],
+        ],
+    )
+
+
+class TestTimeseriesData:
+    def test_merge(self, timeseries_data_1, timeseries_data_2):
+        timeseries_data_1_records_len = len(timeseries_data_1.records)
+        timeseries_data_1_columns_len = len(timeseries_data_1.columns)
+        timeseries_data_2_records_len = len(timeseries_data_2.records)
+        timeseries_data_2_columns_len = len(timeseries_data_2.columns)
+        expected_merged_record_len = len(timeseries_data_1.records[0]) + len(
+            timeseries_data_2.records[0][1:]
+        )
+
+        merged = timeseries_data_1.merge(timeseries_data_2)
+
+        assert merged is timeseries_data_1
+        assert (
+            len(merged.records)
+            == timeseries_data_1_records_len
+            == timeseries_data_2_records_len
+        )
+        assert (
+            len(merged.columns)
+            == timeseries_data_1_columns_len + timeseries_data_2_columns_len - 1
+        )
+
+        for r in merged.records:
+            assert len(r) == expected_merged_record_len
+
+    def test_merge_raises_number_of_records_mismatch(
+        self, timeseries_data_1, timeseries_data_2
+    ):
+        timeseries_data_2.records = timeseries_data_2.records[1:]
+        with pytest.raises(
+            ValueError, match="Number of records in both instances has to be the same"
+        ):
+            timeseries_data_1.merge(timeseries_data_2)
+
+    def test_merge_raises_mismatched_timestamps(
+        self, timeseries_data_1, timeseries_data_2, timestamp
+    ):
+        timeseries_data_2.records[0][0] = (timestamp - timedelta(days=1)).isoformat()
+
+        with pytest.raises(ValueError, match="mismatched timestamps"):
+            timeseries_data_1.merge(timeseries_data_2)