diff --git a/src/TrashLib.Tests/Sonarr/SonarrCompatibilityTest.cs b/src/TrashLib.Tests/Sonarr/SonarrCompatibilityTest.cs
index 568106de..302118db 100644
--- a/src/TrashLib.Tests/Sonarr/SonarrCompatibilityTest.cs
+++ b/src/TrashLib.Tests/Sonarr/SonarrCompatibilityTest.cs
@@ -9,6 +9,7 @@ using NSubstitute;
 using NUnit.Framework;
 using TestLibrary.AutoFixture;
 using TrashLib.ExceptionTypes;
+using TrashLib.Services.Radarr.Config;
 using TrashLib.Services.Sonarr;
 using TrashLib.Services.Sonarr.Api;
 using TrashLib.Services.Sonarr.Api.Objects;
@@ -152,4 +153,40 @@ public class SonarrCompatibilityTest
 
         await act.Should().NotThrowAsync();
     }
+
+    [Test, AutoMockData]
+    public async Task Failure_when_custom_formats_used_with_sonarr_v3(
+        [Frozen] ISonarrApi api,
+        [Frozen(Matching.ImplementedInterfaces)] SonarrCompatibility compatibility,
+        ReleaseProfileUpdater updater)
+    {
+        api.GetVersion().Returns(new Version(3, 9));
+
+        var config = new SonarrConfiguration
+        {
+            CustomFormats = new List<CustomFormatConfig> {new()}
+        };
+
+        var act = () => updater.Process(false, config);
+
+        await act.Should().ThrowAsync<VersionException>().WithMessage("Sonarr v3*custom format*use*v4*");
+    }
+
+    [Test, AutoMockData]
+    public async Task No_failure_when_custom_formats_used_with_sonarr_v4(
+        [Frozen] ISonarrApi api,
+        [Frozen(Matching.ImplementedInterfaces)] SonarrCompatibility compatibility,
+        ReleaseProfileUpdater updater)
+    {
+        api.GetVersion().Returns(new Version(4, 0));
+
+        var config = new SonarrConfiguration
+        {
+            CustomFormats = new List<CustomFormatConfig> {new()}
+        };
+
+        var act = () => updater.Process(false, config);
+
+        await act.Should().NotThrowAsync();
+    }
 }
diff --git a/src/TrashLib/Services/Sonarr/ReleaseProfile/ReleaseProfileUpdater.cs b/src/TrashLib/Services/Sonarr/ReleaseProfile/ReleaseProfileUpdater.cs
index 4099974d..12299376 100644
--- a/src/TrashLib/Services/Sonarr/ReleaseProfile/ReleaseProfileUpdater.cs
+++ b/src/TrashLib/Services/Sonarr/ReleaseProfile/ReleaseProfileUpdater.cs
@@ -206,9 +206,13 @@ public class ReleaseProfileUpdater : IReleaseProfileUpdater
                 $"required version of {_compatibility.MinimumVersion} to use this program");
         }
 
-        if (capabilities.SupportsCustomFormats && config.ReleaseProfiles.Any())
+        switch (capabilities.SupportsCustomFormats)
         {
-            throw new VersionException("Sonarr v4 does not support Release Profiles. Please use Sonarr v3 instead.");
+            case true when config.ReleaseProfiles.Any():
+                throw new VersionException("Sonarr v4 does not support Release Profiles. Please use Sonarr v3 instead.");
+
+            case false when config.CustomFormats.Any():
+                throw new VersionException("Sonarr v3 does not support Custom Formats. Please use Sonarr v4 instead.");
         }
     }