Not all code paths return a value

I've got a function that is returning this error. I can't see where the path is terminating without returning a value. Maybe after numberOfCells++;, but shouldn't it just continue to the next iteration of the for loop?
.

Context: this function should be counting the number of empty cells in a rectangle with the bounds tempStartX, tempStartY and tempEndX, tempEndY by cycling through each cell and checking for empty cells. If it finds any non-empty cells it returns a zero (there is a reason for this). When it reaches the last cell it returns the number of cells. TileType is a 2D array storing the game object that is in each cell.

 public int NumberOfExecutableCells_SubModule (int tempStartX, int tempEndX, int tempStartY, int tempEndY){
 
 int numberOfCells = 0;

 for (int row = tempStartY; row > tempEndY ; row--){             // Cycle through rows
 for (int column = tempStartX; column < tempEndX; column++) {    // Cycle through columns

 if (TileType[column,row].name != "empty(Clone)"){               // If this cell is not empty
     return 0;                                                   // Return zero for # of empty cells
                 }
     else{                                                       // Otherwise if this cell is empty
         numberOfCells++;                                        // Add 1 to the # of empty cells
         if (row == tempEndY && column == tempEndX){             // If this is the last cell
             return numberOfCells;                               // Return the number of empty cells
                 }
             }
         }
     }
 }