diff --git a/src/core/operators/dyn_op.ml b/src/core/operators/dyn_op.ml
index 01c60d48ac..a290e71070 100644
--- a/src/core/operators/dyn_op.ml
+++ b/src/core/operators/dyn_op.ml
@@ -28,7 +28,6 @@ class dyn ~init ~track_sensitive ~infallible ~self_sync ~merge next_fn =
     val mutable activation = []
     val current_source : Source.source option Atomic.t = Atomic.make init
     method current_source = Atomic.get current_source
-    val mutable last_select = Unix.gettimeofday ()
 
     method private no_source =
       if infallible then
@@ -48,29 +47,35 @@ class dyn ~init ~track_sensitive ~infallible ~self_sync ~merge next_fn =
       s#wake_up
 
     method private exchange s =
-      self#log#info "Switching to source %s" s#id;
-      self#prepare s;
-      Atomic.set current_source (Some s);
-      if s#is_ready then Some s else self#no_source
+      match self#current_source with
+        | Some s' when s == s' -> Some s
+        | _ ->
+            self#log#info "Switching to source %s" s#id;
+            self#prepare s;
+            Atomic.set current_source (Some s);
+            if s#is_ready then Some s else self#no_source
 
     method private get_next reselect =
       self#mutexify
         (fun () ->
-          last_select <- Unix.gettimeofday ();
           let s =
             Lang.apply next_fn [] |> Lang.to_option |> Option.map Lang.to_source
           in
-          match s with
-            | None -> (
-                match self#current_source with
-                  | Some s
-                    when self#can_reselect
-                           ~reselect:
-                             (match reselect with `Force -> `Ok | v -> v)
-                           s ->
-                      Some s
-                  | _ -> self#no_source)
-            | Some s -> self#exchange s)
+          match (s, self#current_source) with
+            | None, Some s
+              when self#can_reselect
+                     ~reselect:(match reselect with `Force -> `Ok | v -> v)
+                     s ->
+                Some s
+            | Some s, Some s' when s == s' ->
+                if
+                  self#can_reselect
+                    ~reselect:(match reselect with `Force -> `Ok | v -> v)
+                    s
+                then Some s
+                else self#no_source
+            | Some s, _ -> self#exchange s
+            | _ -> self#no_source)
         ()
 
     method private get_source ~reselect () =
diff --git a/src/core/sources/debug_sources.ml b/src/core/sources/debug_sources.ml
index db0d0594c7..a8dec07838 100644
--- a/src/core/sources/debug_sources.ml
+++ b/src/core/sources/debug_sources.ml
@@ -58,4 +58,28 @@ let _ =
     ~descr:
       "A source that errors during its initialization phase, used for testing \
        and debugging." ~flags:[`Experimental] ~return_t [] (fun _ ->
-      (new fail_init :> Source.source))
+      new fail_init)
+
+class is_ready s =
+  object (self)
+    inherit Source.operator ~name:"is_ready" [s]
+    method seek_source = (self :> Source.source)
+    method fallible = true
+    method private can_generate_frame = true
+    method self_sync = (`Static, None)
+    method remaining = 0
+    method abort_track = ()
+    method generate_frame = if s#is_ready then s#get_frame else self#empty_frame
+  end
+
+let _ =
+  let return_t = Lang.frame_t (Lang.univ_t ()) Frame.Fields.empty in
+  Lang.add_operator ~base:Modules.debug "is_ready" ~category:`Input
+    ~descr:
+      "A source that always produces an empty frame when the underlying source \
+       is not ready, used for testing and debugging."
+    ~flags:[`Experimental] ~return_t
+    [("", Lang.source_t return_t, None, None)]
+    (fun p ->
+      let s = Lang.to_source (List.assoc "" p) in
+      new is_ready s)
diff --git a/tests/regression/GH4159.liq b/tests/regression/GH4159.liq
new file mode 100644
index 0000000000..e17b86607d
--- /dev/null
+++ b/tests/regression/GH4159.liq
@@ -0,0 +1,5 @@
+s = debug.is_ready(sine(duration=0.2))
+
+s = source.dynamic(track_sensitive=false, {s})
+
+output.dummy(fallible=true, on_stop=test.pass, s)
diff --git a/tests/regression/dune.inc b/tests/regression/dune.inc
index e8c347a689..25850015bc 100644
--- a/tests/regression/dune.inc
+++ b/tests/regression/dune.inc
@@ -735,6 +735,22 @@
   (:run_test ../run_test.exe))
  (action (run %{run_test} GH4144.liq liquidsoap %{test_liq} GH4144.liq)))
   
+(rule
+ (alias citest)
+ (package liquidsoap)
+ (deps
+  GH4159.liq
+  ../media/all_media_files
+  ../../src/bin/liquidsoap.exe
+  ../streams/file1.png
+  ../streams/file1.mp3
+  ./theora-test.mp4
+  (package liquidsoap)
+  (source_tree ../../src/libs)
+  (:test_liq ../test.liq)
+  (:run_test ../run_test.exe))
+ (action (run %{run_test} GH4159.liq liquidsoap %{test_liq} GH4159.liq)))
+  
 (rule
  (alias citest)
  (package liquidsoap)