diff --git a/tests/operations/index.mjs b/tests/operations/index.mjs
index c12c2710..ed6a2616 100644
--- a/tests/operations/index.mjs
+++ b/tests/operations/index.mjs
@@ -159,6 +159,7 @@ import "./tests/SeqUtils.mjs";
 import "./tests/SetDifference.mjs";
 import "./tests/SetIntersection.mjs";
 import "./tests/SetUnion.mjs";
+import "./tests/ShowOnMap.mjs";
 import "./tests/Shuffle.mjs";
 import "./tests/SIGABA.mjs";
 import "./tests/SM2.mjs";
diff --git a/tests/operations/tests/ShowOnMap.mjs b/tests/operations/tests/ShowOnMap.mjs
new file mode 100644
index 00000000..f9af64ef
--- /dev/null
+++ b/tests/operations/tests/ShowOnMap.mjs
@@ -0,0 +1,39 @@
+/**
+ * Show on map tests
+ *
+ * @author Leon Zandman [leon@wirwar.com]
+ *
+ * @copyright Crown Copyright 2026
+ * @license Apache-2.0
+ */
+import TestRegister from "../../lib/TestRegister.mjs";
+
+TestRegister.addTests([
+    {
+        name: "Show on map: valid co-ordinate pair",
+        input: "51.5007, -0.1246",
+        // The presented output is the Leaflet map HTML; just check the co-ordinates made it through.
+        expectedMatch: /51\.5007,-0\.1246/,
+        recipeConfig: [
+            {
+                op: "Show on map",
+                args: [13, "Auto", "Auto"]
+            },
+        ],
+    },
+    {
+        // Regression test: a comma-separated input with the delimiter set to "\n" used to be
+        // mis-detected as a single Degrees Decimal Minutes value (1° 24' = 1.4°), producing a single
+        // co-ordinate. That single value was then passed to Leaflet's setView([1.4], ...), throwing
+        // an uncaught "Cannot read properties of null (reading 'lat')" TypeError in the browser.
+        name: "Show on map: single value is rejected with a helpful error",
+        input: "1, 24",
+        expectedOutput: "Could not show co-ordinates '1.4' on the map. Expected a latitude and longitude pair - check that the input format and delimiter are correct.",
+        recipeConfig: [
+            {
+                op: "Show on map",
+                args: [13, "Auto", "\\n"]
+            },
+        ],
+    },
+]);