diff --git a/app/src/main/java/gui/SudokuController.java b/app/src/main/java/gui/SudokuController.java
index a283ab8..0d91eed 100644
--- a/app/src/main/java/gui/SudokuController.java
+++ b/app/src/main/java/gui/SudokuController.java
@@ -127,6 +127,15 @@ public class SudokuController {
 
             // Update the model and view
             model.set(row, col, value);
+
+            // Warn if the board is not solvable (e.g. if the user has made a mistake)
+            // This is very messy, error prone and computationally expensive
+            if(!model.isSolvable()) {
+                model.set(row, col, 0);
+                view.showErrorMessage("Illegal move. The board is not solvable.");
+            }
+
+            // Sync the view with the model
             view.updateView(model.getBoard());
         }
     }
diff --git a/app/src/test/java/sudoku/SolverTest.java b/app/src/test/java/sudoku/SolverTest.java
index 3a5833b..7000140 100644
--- a/app/src/test/java/sudoku/SolverTest.java
+++ b/app/src/test/java/sudoku/SolverTest.java
@@ -34,7 +34,8 @@ class SolverTest {
         assertTrue(solver.isLegal(0, 0, 1));
         solver.set(0, 0, 1);
 
-        IntStream.range(0, 9).forEach(i -> {
+        // Start from one, since setting the same value is legal
+        IntStream.range(1, 9).forEach(i -> {
             assertFalse(solver.isLegal(0, i, 1));
             assertFalse(solver.isLegal(i, 0, 1));
         });