diff --git a/tests/test_api.py b/tests/test_api.py
index e021ef6..2d075f3 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -55,8 +55,8 @@ def _assert_coordinate(entry, expected_json_output, tolerance=1e-6):
     client = TestClient(app)
     response = client.get(entry)
     result = response.json()
-    print(expected_json_output)
-    print(result)
+    print("expected_output:", expected_json_output)
+    print("results        :", result)
     for key in expected_json_output.keys():
         if key not in result.keys():
             raise AssertionError
@@ -316,6 +316,20 @@ def test_combined_epsg_codes(api_all):
     }
     _assert_coordinate(api_entry, expected, tolerance=0.01)
 
+def test_conversion_to_3d(api_all):
+    """
+    Test that a 2D CRS is correctly promoted to 3D when used in
+    combination with a 3D CRS.
+
+    """
+    api_entry = f"/{api_all}/trans/EPSG:4258/EPSG:4258+5799/55.5,11.5,0"
+    expected = {
+        "v1": 55.5,
+        "v2": 11.5,
+        "v3": -37.4190,
+        "v4": None,
+    }
+    _assert_coordinate(api_entry, expected, tolerance=0.01)
 
 def test_crs_return_srid(api_from_v1_1):
     """
diff --git a/webproj/api.py b/webproj/api.py
index cb6aaaf..218c044 100644
--- a/webproj/api.py
+++ b/webproj/api.py
@@ -14,11 +14,11 @@
 from fastapi.middleware.cors import CORSMiddleware
 from pydantic import BaseModel
 import pyproj
-from pyproj.transformer import Transformer, AreaOfInterest
+from pyproj.transformer import Transformer, AreaOfInterest, CRS
 
 from webproj.middleware import ProxyHeaderMiddleware
 
-__VERSION__ = "1.2.3"
+__VERSION__ = "1.2.4"
 
 if "WEBPROJ_LIB" in os.environ:
     pyproj.datadir.append_data_dir(os.environ["WEBPROJ_LIB"])
@@ -182,8 +182,13 @@ def __init__(self, src, dst):
                 dst_hub = "EPSG:4909"
 
             try:
+                # Explicit promotion to 3D CRS's to ensure vertical
+                # transformations are picked up correctly.
+                # Tested in test_conversion_to_3d().
                 self.epsg_pipeline = Transformer.from_crs(
-                    src, dst_hub, area_of_interest=region
+                    crs_from=CRS(src).to_3d(),
+                    crs_to=CRS(dst_hub).to_3d(),
+                    area_of_interest=region,
                 )
             except RuntimeError as error:
                 raise ValueError("Invalid CRS identifier") from error