diff --git a/app/src/main/java/gui/SudokuController.java b/app/src/main/java/gui/SudokuController.java
index 5528349..a95f1a4 100644
--- a/app/src/main/java/gui/SudokuController.java
+++ b/app/src/main/java/gui/SudokuController.java
@@ -94,24 +94,31 @@ public class SudokuController {
             int row = view.getSelectedRow();
             int col = view.getSelectedColumn();
 
+            // The value to be inserted into the cell
+            // Zero inicates an empty cell
+            // Negative values are invalid
             int value = 0;
 
             String cellValue = view.getCellValue(row, col);
+
+            // We need to check for null and empty string
             if (cellValue == null || cellValue.equals("")) {
                 value = 0;
             } else {
                 try {
                     value = Integer.parseInt(cellValue);
                 } catch (NumberFormatException ex) {
-                    value = 0;
+                    value = -1;
                 }
             }
 
-            // If the input is invalid
-            if (!model.isLegal(row, col, value)) {
+            // If the input is invalid, value < 0 indicates parse error
+            if (!model.isLegal(row, col, value) || value < 0) {
                 value = 0;
                 view.showErrorMessage("Invalid input. Try again.");
             }
+
+            // Update the model and view
             model.set(row, col, value);
             view.updateView(model.getBoard());
         }