diff --git a/docs/release-notes.md b/docs/release-notes.md
index 9047ae88..63343d0a 100644
--- a/docs/release-notes.md
+++ b/docs/release-notes.md
@@ -1,4 +1,8 @@
 # Release notes
+## Upcoming release
+* For players:
+  * Fixed error when a mod makes invalid changes to an NPC schedule.
+
 ## 2.9.1
 * For players:
   * Fixed crash in SMAPI 2.9 when constructing certain buildings.
diff --git a/src/SMAPI/Framework/ContentCoordinator.cs b/src/SMAPI/Framework/ContentCoordinator.cs
index 08a32a9b..7e256939 100644
--- a/src/SMAPI/Framework/ContentCoordinator.cs
+++ b/src/SMAPI/Framework/ContentCoordinator.cs
@@ -81,7 +81,7 @@ namespace StardewModdingAPI.Framework
             this.ContentManagers.Add(
                 this.MainContentManager = new GameContentManager("Game1.content", serviceProvider, rootDirectory, currentCulture, this, monitor, reflection, this.OnDisposing)
             );
-            this.CoreAssets = new CoreAssetPropagator(this.MainContentManager.AssertAndNormaliseAssetName, reflection);
+            this.CoreAssets = new CoreAssetPropagator(this.MainContentManager.AssertAndNormaliseAssetName, reflection, monitor);
         }
 
         /// <summary>Get a new content manager which handles reading files from the game content folder with support for interception.</summary>
diff --git a/src/SMAPI/Metadata/CoreAssetPropagator.cs b/src/SMAPI/Metadata/CoreAssetPropagator.cs
index 90629d7f..4667be7e 100644
--- a/src/SMAPI/Metadata/CoreAssetPropagator.cs
+++ b/src/SMAPI/Metadata/CoreAssetPropagator.cs
@@ -31,6 +31,9 @@ namespace StardewModdingAPI.Metadata
         /// <summary>Simplifies access to private game code.</summary>
         private readonly Reflector Reflection;
 
+        /// <summary>Encapsulates monitoring and logging.</summary>
+        private readonly IMonitor Monitor;
+
 
         /*********
         ** Public methods
@@ -38,10 +41,12 @@ namespace StardewModdingAPI.Metadata
         /// <summary>Initialise the core asset data.</summary>
         /// <param name="getNormalisedPath">Normalises an asset key to match the cache key.</param>
         /// <param name="reflection">Simplifies access to private code.</param>
-        public CoreAssetPropagator(Func<string, string> getNormalisedPath, Reflector reflection)
+        /// <param name="monitor">Encapsulates monitoring and logging.</param>
+        public CoreAssetPropagator(Func<string, string> getNormalisedPath, Reflector reflection, IMonitor monitor)
         {
             this.GetNormalisedPath = getNormalisedPath;
             this.Reflection = reflection;
+            this.Monitor = monitor;
         }
 
         /// <summary>Reload one of the game's core assets (if applicable).</summary>
@@ -619,6 +624,11 @@ namespace StardewModdingAPI.Metadata
             {
                 // reload schedule
                 villager.Schedule = villager.getSchedule(Game1.dayOfMonth);
+                if (villager.Schedule == null)
+                {
+                    this.Monitor.Log($"A mod set an invalid schedule for {villager.Name ?? key}, so the NPC may not behave correctly.", LogLevel.Warn);
+                    return true;
+                }
 
                 // switch to new schedule if needed
                 int lastScheduleTime = villager.Schedule.Keys.Where(p => p <= Game1.timeOfDay).OrderByDescending(p => p).FirstOrDefault();