diff --git a/app/Draft/Commands/GenerateSlicePool.php b/app/Draft/Commands/GenerateSlicePool.php
index 2df6eab..c0723e6 100644
--- a/app/Draft/Commands/GenerateSlicePool.php
+++ b/app/Draft/Commands/GenerateSlicePool.php
@@ -208,7 +208,13 @@ private function validateTileSelection(array $tileIds): bool
     private function slicesFromCustomSlices(): array
     {
         return array_map(function (array $sliceData) {
-            $tileData = array_map(fn ($tileId) => $this->tileData[$tileId], $sliceData);
+            $tileData = [];
+            foreach ($sliceData as $tileId) {
+                if (! isset($this->tileData[$tileId])) {
+                    throw InvalidDraftSettingsException::unknownTileInCustomSlice($tileId);
+                }
+                $tileData[] = $this->tileData[$tileId];
+            }
 
             return new Slice($tileData);
         }, $this->settings->customSlices);
diff --git a/app/Draft/Commands/GenerateSlicePoolTest.php b/app/Draft/Commands/GenerateSlicePoolTest.php
index 40eca14..b5fad29 100644
--- a/app/Draft/Commands/GenerateSlicePoolTest.php
+++ b/app/Draft/Commands/GenerateSlicePoolTest.php
@@ -256,6 +256,23 @@ public function itCanReturnCustomSlices(): void
         }
     }
 
+    #[Test]
+    public function itThrowsErrorWhenCustomSliceContainsIncorrectTileNumber(): void
+    {
+        $customSlices = [
+            ['64', '33AEZD', '42', '67', '59'],
+        ];
+
+        $generator = new GenerateSlicePool(DraftSettingsFactory::make([
+            'numberOfSlices' => 4,
+            'customSlices' => $customSlices,
+        ]));
+
+        $this->expectException(InvalidDraftSettingsException::class);
+
+        $slices = $generator->handle();
+    }
+
     #[Test]
     public function itGivesUpIfSettingsAreImpossible(): void
     {
diff --git a/app/Draft/Exceptions/InvalidDraftSettingsException.php b/app/Draft/Exceptions/InvalidDraftSettingsException.php
index 6b26754..a94ffd8 100644
--- a/app/Draft/Exceptions/InvalidDraftSettingsException.php
+++ b/app/Draft/Exceptions/InvalidDraftSettingsException.php
@@ -34,6 +34,11 @@ public static function notEnoughFactionsForPlayers(): self
         return new self('Cannot have less factions than players');
     }
 
+    public static function unknownTileInCustomSlice($id): self
+    {
+        return new self('Custom slices contain unknown tile number:' . $id);
+    }
+
     public static function notEnoughTilesForSlices(float $maxSlices): self
     {
         return new self(sprintf('This selection of tilesets only supports %d slices', $maxSlices));
diff --git a/app/Http/RequestHandlers/HandleGenerateDraftRequest.php b/app/Http/RequestHandlers/HandleGenerateDraftRequest.php
index 0df8c58..828923c 100644
--- a/app/Http/RequestHandlers/HandleGenerateDraftRequest.php
+++ b/app/Http/RequestHandlers/HandleGenerateDraftRequest.php
@@ -61,7 +61,7 @@ private function settingsFromRequest(): Settings
                 $slice = [];
                 $t = explode(',', $s);
                 foreach ($t as $tile) {
-                    $tile = trim($tile);
+                    $tile = ltrim(trim($tile), '0');
                     $slice[] = $tile;
                 }
                 $customSlices[] = $slice;
diff --git a/app/Http/RequestHandlers/HandleGenerateDraftRequestTest.php b/app/Http/RequestHandlers/HandleGenerateDraftRequestTest.php
index e234439..b6e62ac 100644
--- a/app/Http/RequestHandlers/HandleGenerateDraftRequestTest.php
+++ b/app/Http/RequestHandlers/HandleGenerateDraftRequestTest.php
@@ -76,13 +76,13 @@ public static function settingsPayload()
         ];
         yield 'Custom Slices' => [
             'postData' => [
-                'custom_slices' => "1,2,3,4,5\n6,7,8,9,10\n11,12,13,14,15",
+                'custom_slices' => "01,2,03,4,5\n6,7,8,9,10\n11,012,13,014,15A",
             ],
             'field' => 'customSlices',
             'expected' => [
                ['1', '2', '3', '4', '5'],
                ['6', '7', '8', '9', '10'],
-               ['11', '12', '13', '14', '15'],
+               ['11', '12', '13', '14', '15A'],
             ],
             'expectedWhenNotSet' => [],
         ];