diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/UnsTreeService.cs b/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/UnsTreeService.cs
index a6a5e183..7b3bc51f 100644
--- a/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/UnsTreeService.cs
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/UnsTreeService.cs
@@ -523,6 +523,9 @@ public sealed class UnsTreeService(IDbContextFactory<OtOpcUaConfigDbContext> dbF
             return new UnsMutationResult(false, "TagConfig is not valid JSON.");
         }
 
+        if (!await db.Equipment.AnyAsync(e => e.EquipmentId == equipmentId, ct))
+            return new UnsMutationResult(false, $"Equipment '{equipmentId}' not found.");
+
         var equipmentCluster = await ResolveEquipmentClusterAsync(db, equipmentId, ct);
         var guard = await CheckTagDriverGuardAsync(db, input.DriverInstanceId, equipmentCluster, ct);
         if (guard is not null)
@@ -648,7 +651,7 @@ public sealed class UnsTreeService(IDbContextFactory<OtOpcUaConfigDbContext> dbF
             using var _ = System.Text.Json.JsonDocument.Parse(json);
             return true;
         }
-        catch
+        catch (System.Text.Json.JsonException)
         {
             return false;
         }
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/UnsTreeServiceTagTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/UnsTreeServiceTagTests.cs
index eb8f9d6d..68183c7e 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/UnsTreeServiceTagTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/UnsTreeServiceTagTests.cs
@@ -219,6 +219,23 @@ public sealed class UnsTreeServiceTagTests
         result.Error.ShouldBe("Tag 'TAG-1' already exists.");
     }
 
+    /// <summary>Creating a tag for an equipment id that does not exist returns a not-found error.</summary>
+    [Fact]
+    public async Task CreateTag_unresolvable_equipment_returns_error()
+    {
+        var (service, dbName) = Fresh();
+        SeedHierarchyAndDrivers(dbName, equipmentCluster: "MAIN", seedEquipmentDriver: true);
+
+        var result = await service.CreateTagAsync("EQ-NOPE", Input("TAG-1", "speed", "DRV-EQ"));
+
+        result.Ok.ShouldBeFalse();
+        result.Error.ShouldNotBeNull();
+        result.Error.ShouldContain("not found");
+
+        using var db = UnsTreeTestDb.CreateNamed(dbName);
+        db.Tags.Any(t => t.TagId == "TAG-1").ShouldBeFalse();
+    }
+
     /// <summary>Creating a tag whose Name already exists on the same equipment is blocked.</summary>
     [Fact]
     public async Task CreateTag_duplicate_name_on_equipment_blocked()