diff --git a/build.gradle b/build.gradle
index da67b39dae..81332a0b83 100644
--- a/build.gradle
+++ b/build.gradle
@@ -73,4 +73,7 @@ shadowJar {
 
 run {
     standardInput = System.in
+    // Java ignores assert statements unless told otherwise, so turn them on when
+    // running the app from Gradle. Tests already run with assertions on.
+    enableAssertions = true
 }
diff --git a/src/main/java/ted/MainWindow.java b/src/main/java/ted/MainWindow.java
index 3ded5d5653..8658e605ba 100644
--- a/src/main/java/ted/MainWindow.java
+++ b/src/main/java/ted/MainWindow.java
@@ -59,6 +59,7 @@ public void setTed(Ted ted) {
      */
     @FXML
     private void handleUserInput() {
+        assert ted != null : "Main calls setTed before showing the window, so input always has a Ted";
         String input = userInput.getText();
         String response = ted.getResponse(input);
         if (response.isEmpty()) {
diff --git a/src/main/java/ted/Parser.java b/src/main/java/ted/Parser.java
index 5740763887..7817ce3e3d 100644
--- a/src/main/java/ted/Parser.java
+++ b/src/main/java/ted/Parser.java
@@ -59,6 +59,8 @@ private Parser() {
      * @throws TedException if the command is unknown or its details are unusable.
      */
     public static Command parse(String input) throws TedException {
+        assert !input.isBlank() : "Ted skips blank input, so it never reaches the parser";
+
         // Splitting into at most two parts keeps the command word exact, so that
         // "todos" is not mistaken for "todo", while leaving the rest untouched.
         String[] parts = input.split(" ", 2);
diff --git a/src/main/java/ted/Storage.java b/src/main/java/ted/Storage.java
index 2a01d32fba..adf6606cf8 100644
--- a/src/main/java/ted/Storage.java
+++ b/src/main/java/ted/Storage.java
@@ -139,6 +139,8 @@ public int getSkippedLineCount() {
      * @return the rebuilt task, or {@code null} if the line is not in the expected format.
      */
     private static Task parseLine(String line) {
+        assert !line.isBlank() : "load() skips blank lines, so they are never parsed";
+
         // A first, unlimited split just to read the type icon safely: even a
         // corrupted line must have at least its icon before anything can be parsed.
         String icon = line.split(FIELD_SEPARATOR_REGEX)[0];
@@ -172,7 +174,7 @@ private static Task parseLine(String line) {
                             parseSavedDateTime(fields[3])),
                             isDone);
                 default:
-                    // Unreachable: fieldCountFor accepts only T, D and E.
+                    assert false : "unreachable, as fieldCountFor accepts only T, D and E";
                     return null;
             }
         } catch (DateTimeParseException e) {
diff --git a/src/main/java/ted/Ted.java b/src/main/java/ted/Ted.java
index 53b347989b..0e16f88082 100644
--- a/src/main/java/ted/Ted.java
+++ b/src/main/java/ted/Ted.java
@@ -123,6 +123,7 @@ public String getResponse(String input) {
             return ui.flush();
         }
 
+        assert tasks != null : "the constructor sets up a task list even when loading fails";
         try {
             command.execute(tasks, ui, storage);
         } catch (TedException e) {
diff --git a/src/main/java/ted/task/TaskList.java b/src/main/java/ted/task/TaskList.java
index d3ba0b7cc2..dd3c4dae67 100644
--- a/src/main/java/ted/task/TaskList.java
+++ b/src/main/java/ted/task/TaskList.java
@@ -28,6 +28,7 @@ public TaskList() {
      * @param tasks the tasks to start with.
      */
     public TaskList(List<Task> tasks) {
+        assert tasks.stream().allMatch(task -> task != null) : "Storage.load() never adds null";
         this.tasks = new ArrayList<>(tasks);
     }