diff --git a/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayer.java b/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayer.java
index 99144fbddc..06d4bd165d 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayer.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayer.java
@@ -436,6 +436,7 @@ public interface ExoPlayer extends Player {
      * @param timeoutMs The time limit in milliseconds.
      */
     public Builder experimentalSetForegroundModeTimeoutMs(long timeoutMs) {
+      Assertions.checkState(!buildCalled);
       setForegroundModeTimeoutMs = timeoutMs;
       return this;
     }
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java b/library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java
index 990de0f9e1..e712d4396d 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java
@@ -263,6 +263,7 @@ public class SimpleExoPlayer extends BasePlayer
      * @param timeoutMs The time limit in milliseconds.
      */
     public Builder experimentalSetForegroundModeTimeoutMs(long timeoutMs) {
+      Assertions.checkState(!buildCalled);
       foregroundModeTimeoutMs = timeoutMs;
       return this;
     }