diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3846159c..1011a3b7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,11 +6,15 @@ All notable changes to **NCronJob** will be documented in this file. The project
 
 ## [Unreleased]
 
+### Changed
+
+- Prevent `InstantJobRegistry` from altering the content of `JobRegistry`. Fixed in [#131](https://github.com/NCronJob-Dev/NCronJob/issues/131), by [@nulltoken](https://github.com/nulltoken).
+
 ## [3.3.4] - 2024-11-03
 
 ### Fixes
 
-- Ensure multiples schedules of the same job with different chains of dependent jobs are properly processed. Identified in [#108](https://github.com/NCronJob-Dev/NCronJob/issues/107), by [@nulltoken](https://github.com/nulltoken).
+- Ensure multiples schedules of the same job with different chains of dependent jobs are properly processed. Identified in [#108](https://github.com/NCronJob-Dev/NCronJob/issues/108), by [@nulltoken](https://github.com/nulltoken).
 
 - Teach `IRuntimeJobRegistry.RemoveJob()` to clean up potential dependent jobs. Fixes [#107](https://github.com/NCronJob-Dev/NCronJob/issues/107), by [@nulltoken](https://github.com/nulltoken).
 
diff --git a/src/NCronJob/Registry/IInstantJobRegistry.cs b/src/NCronJob/Registry/IInstantJobRegistry.cs
index 3ddd03bb..fe7703ac 100644
--- a/src/NCronJob/Registry/IInstantJobRegistry.cs
+++ b/src/NCronJob/Registry/IInstantJobRegistry.cs
@@ -236,17 +236,14 @@ private void RunJob<TJob>(DateTimeOffset startDate, object? parameter = null, bo
     {
         using (logger.BeginScope("Triggering RunScheduledJob:"))
         {
-            if (!jobRegistry.IsJobRegistered<TJob>())
+            var jobDefinition = jobRegistry.FindJobDefinition(typeof(TJob));
+
+            if (jobDefinition is null)
             {
                 LogJobNotRegistered(typeof(TJob).Name);
-                var newJobDefinition = new JobDefinition(typeof(TJob), parameter, null, null);
-                jobRegistry.Add(newJobDefinition);
+                jobDefinition = new JobDefinition(typeof(TJob), parameter, null, null);
             }
 
-            var jobDefinition = jobRegistry.FindJobDefinition(typeof(TJob));
-
-            Debug.Assert(jobDefinition != null);
-
             token.Register(() => LogCancellationRequested(parameter));
 
             RunInternal(jobDefinition, parameter, startDate, forceExecution, token);
diff --git a/src/NCronJob/Registry/JobRegistry.cs b/src/NCronJob/Registry/JobRegistry.cs
index 73f4fc97..dc1fdc6e 100644
--- a/src/NCronJob/Registry/JobRegistry.cs
+++ b/src/NCronJob/Registry/JobRegistry.cs
@@ -16,10 +16,6 @@ private readonly Dictionary<JobDefinition, List<DependentJobRegistryEntry>> depe
 
     public IReadOnlyCollection<JobDefinition> GetAllOneTimeJobs() => allJobs.Where(c => c.IsStartupJob).ToList();
 
-    public bool IsJobRegistered<T>() => allJobs.Any(j => j.Type == typeof(T));
-
-    public JobDefinition GetJobDefinition<T>() => allJobs.First(j => j.Type == typeof(T));
-
     public JobDefinition? FindJobDefinition(Type type)
         => allJobs.FirstOrDefault(j => j.Type == type);
 
diff --git a/tests/NCronJob.Tests/NCronJobIntegrationTests.cs b/tests/NCronJob.Tests/NCronJobIntegrationTests.cs
index aa7cef8a..65539ef0 100644
--- a/tests/NCronJob.Tests/NCronJobIntegrationTests.cs
+++ b/tests/NCronJob.Tests/NCronJobIntegrationTests.cs
@@ -69,6 +69,23 @@ public async Task EachJobRunHasItsOwnScope()
         storage.Guids.Distinct().Count().ShouldBe(storage.Guids.Count);
     }
 
+    [Fact]
+    public async Task ExecuteAnInstantJobWithoutPreviousRegistration()
+    {
+        ServiceCollection.AddNCronJob();
+
+        var provider = CreateServiceProvider();
+        await provider.GetRequiredService<IHostedService>().StartAsync(CancellationToken);
+
+        provider.GetRequiredService<IInstantJobRegistry>().RunInstantJob<SimpleJob>();
+
+        var jobFinished = await WaitForJobsOrTimeout(1);
+        jobFinished.ShouldBeTrue();
+
+        var jobRegistry = provider.GetRequiredService<JobRegistry>();
+        jobRegistry.FindJobDefinition(typeof(SimpleJob)).ShouldBeNull();
+    }
+
     [Fact]
     public async Task ExecuteAnInstantJob()
     {