diff --git a/src/Query/Builder.php b/src/Query/Builder.php
index 3e45bd8..3511204 100644
--- a/src/Query/Builder.php
+++ b/src/Query/Builder.php
@@ -201,7 +201,11 @@ public function where(
         string $boolean = 'AND',
     ): static {
         // Nested group: ->where(function($q) { $q->where(...)->orWhere(...); })
-        if (is_callable($column)) {
+        // Only Closures count as nested groups. A plain string column name must
+        // NEVER be treated as callable — column names like 'key', 'list', or
+        // 'count' collide with built-in PHP function names and is_callable()
+        // would wrongly return true for them.
+        if ($column instanceof \Closure) {
             return $this->whereNested($column, $boolean);
         }
 
@@ -535,7 +539,7 @@ public function whereNested(callable $callback, string $boolean = 'AND'): static
      */
     public function whereExists(callable|Builder $subquery, string $boolean = 'AND', bool $not = false): static
     {
-        if (is_callable($subquery)) {
+        if ($subquery instanceof \Closure) {
             $sub = $this->newQuery();
             $subquery($sub);
         } else {
@@ -939,7 +943,10 @@ protected function addJoin(
     ): static {
         $join = new JoinClause($type, $table);
 
-        if (is_callable($firstOrCallback)) {
+        // Only a Closure is the advanced callback form. A plain string is the
+        // first join column — is_callable() would wrongly match column names
+        // like 'key' that collide with built-in PHP function names.
+        if ($firstOrCallback instanceof \Closure) {
             $firstOrCallback($join);
         } else {
             $join->on($firstOrCallback, $operator ?? '=', $second ?? '');
@@ -1190,17 +1197,17 @@ public function get(): Collection
     }
 
     /**
-     * Execute the query and return the first row, or false if none.
+     * Execute the query and return the first row, or null if none.
      * When a hydrator is set, the row is transformed into a model instance.
      *
-     * @return object|false
+     * @return object|null
      */
-    public function first(): object|false
+    public function first(): ?object
     {
         $row = $this->limit(1)->connection->selectOne($this->toSql(), $this->getBindings());
 
-        if ($row === false) {
-            return false;
+        if ($row === false || $row === null) {
+            return null;
         }
 
         if ($this->hydrator !== null) {
@@ -1216,7 +1223,7 @@ public function first(): object|false
      * @param  int|string $id
      * @return object|false
      */
-    public function find(int|string $id): object|false
+    public function find(int|string $id): ?object
     {
         return $this->where($this->primaryKey, '=', $id)->first();
     }
diff --git a/src/Query/Grammars/Grammar.php b/src/Query/Grammars/Grammar.php
index 0a9fa26..934a9a6 100644
--- a/src/Query/Grammars/Grammar.php
+++ b/src/Query/Grammars/Grammar.php
@@ -328,7 +328,13 @@ protected function compileWhereRaw(array $where): string
      */
     protected function compileWhereIn(array $where): string
     {
-        $col         = $this->wrapColumn($where['column']);
+        // An empty IN list is invalid SQL in MySQL/PostgreSQL (`IN ()`).
+        // Semantically `x IN (nothing)` is always false, so emit `1 = 0`.
+        if (count($where['values']) === 0) {
+            return '1 = 0';
+        }
+
+        $col          = $this->wrapColumn($where['column']);
         $placeholders = implode(', ', array_fill(0, count($where['values']), '?'));
 
         return "{$col} IN ({$placeholders})";
@@ -340,6 +346,11 @@ protected function compileWhereIn(array $where): string
      */
     protected function compileWhereNotIn(array $where): string
     {
+        // An empty NOT IN list means "exclude nothing" → always true → `1 = 1`.
+        if (count($where['values']) === 0) {
+            return '1 = 1';
+        }
+
         $col          = $this->wrapColumn($where['column']);
         $placeholders = implode(', ', array_fill(0, count($where['values']), '?'));
 
diff --git a/src/Support/Collection.php b/src/Support/Collection.php
index 786a02d..d4ef566 100644
--- a/src/Support/Collection.php
+++ b/src/Support/Collection.php
@@ -347,7 +347,10 @@ public function isNotEmpty(): bool
      */
     public function contains(callable|string $callbackOrColumn, mixed $value = null): bool
     {
-        if (is_callable($callbackOrColumn)) {
+        // Only a Closure is the callback form. A plain string is always a column
+        // name — is_callable() would wrongly match column names like 'key' or
+        // 'count' that collide with built-in PHP function names.
+        if ($callbackOrColumn instanceof \Closure) {
             foreach ($this->items as $item) {
                 if ($callbackOrColumn($item)) {
                     return true;
diff --git a/tests/Integration/QueryTest.php b/tests/Integration/QueryTest.php
index ef3a651..e049b35 100644
--- a/tests/Integration/QueryTest.php
+++ b/tests/Integration/QueryTest.php
@@ -164,7 +164,7 @@ public function test_first_returns_single_row(): void
 
     public function test_first_returns_null_when_empty(): void
     {
-        $this->assertFalse($this->table()->first());
+        $this->assertNull($this->table()->first());
     }
 
     public function test_value_returns_single_column(): void
diff --git a/tests/Unit/BuilderTest.php b/tests/Unit/BuilderTest.php
index ead4941..b58df44 100644
--- a/tests/Unit/BuilderTest.php
+++ b/tests/Unit/BuilderTest.php
@@ -119,6 +119,24 @@ public function test_where_not_in(): void
         $this->assertSql('SELECT * FROM `users` WHERE `status` NOT IN (?, ?)', $b);
     }
 
+    /**
+     * Regression: an empty IN list produces invalid SQL (`IN ()`) on
+     * MySQL/PostgreSQL. `x IN (nothing)` is always false, so it must
+     * compile to `1 = 0` (and NOT IN to `1 = 1`).
+     */
+    public function test_where_in_empty_array(): void
+    {
+        $b = $this->builder()->whereIn('id', []);
+        $this->assertSql('SELECT * FROM `users` WHERE 1 = 0', $b);
+        $this->assertSame([], $b->getBindings());
+    }
+
+    public function test_where_not_in_empty_array(): void
+    {
+        $b = $this->builder()->whereNotIn('id', []);
+        $this->assertSql('SELECT * FROM `users` WHERE 1 = 1', $b);
+    }
+
     public function test_where_null(): void
     {
         $this->assertSql(
@@ -160,6 +178,33 @@ public function test_where_nested_group(): void
         );
     }
 
+    /**
+     * Regression: column names that collide with built-in PHP function names
+     * (key, list, count, current, ...) were wrongly detected as callables by
+     * is_callable(), routing them into whereNested() and triggering
+     * "Calling key() on an object is deprecated". Only Closures are nested
+     * groups now — a plain string is always a column name.
+     */
+    public function test_where_with_php_function_named_column(): void
+    {
+        $b = $this->builder()->where('key', 'footer_settings');
+        $this->assertSql('SELECT * FROM `users` WHERE `key` = ?', $b);
+        $this->assertSame(['footer_settings'], $b->getBindings());
+    }
+
+    public function test_where_with_count_column(): void
+    {
+        $b = $this->builder()->where('count', '>', 5);
+        $this->assertSql('SELECT * FROM `users` WHERE `count` > ?', $b);
+    }
+
+    public function test_nested_group_still_works_with_closure(): void
+    {
+        // A real Closure must still produce a nested group
+        $b = $this->builder()->where(fn($q) => $q->where('a', 1)->orWhere('b', 2));
+        $this->assertSql('SELECT * FROM `users` WHERE (`a` = ? OR `b` = ?)', $b);
+    }
+
     public function test_where_raw(): void
     {
         $b = $this->builder()->whereRaw('age > 18 AND active = 1');
@@ -297,4 +342,4 @@ public function test_true_false_shorthands(): void
             $this->builder()->false('active'),
         );
     }
-}
+}
\ No newline at end of file
diff --git a/tests/Unit/CollectionTest.php b/tests/Unit/CollectionTest.php
index 448c3b6..18bf6f9 100644
--- a/tests/Unit/CollectionTest.php
+++ b/tests/Unit/CollectionTest.php
@@ -160,6 +160,38 @@ public function test_reduce(): void
         $this->assertSame(270.0, $total);
     }
 
+    // -----------------------------------------------------------------------
+    // contains
+    // -----------------------------------------------------------------------
+
+    public function test_contains_with_closure(): void
+    {
+        $this->assertTrue($this->makeItems()->contains(fn($u) => $u->role === 'admin'));
+        $this->assertFalse($this->makeItems()->contains(fn($u) => $u->role === 'ghost'));
+    }
+
+    public function test_contains_with_column_and_value(): void
+    {
+        $this->assertTrue($this->makeItems()->contains('name', 'Alice'));
+        $this->assertFalse($this->makeItems()->contains('name', 'Nobody'));
+    }
+
+    /**
+     * Regression: contains('key', ...) must compare against the 'key' column,
+     * not treat 'key' as a callable. is_callable('key') returns true because
+     * key() is a built-in PHP function.
+     */
+    public function test_contains_with_php_function_named_column(): void
+    {
+        $items = Collection::make([
+            ['id' => 1, 'key' => 'footer'],
+            ['id' => 2, 'key' => 'header'],
+        ]);
+
+        $this->assertTrue($items->contains('key', 'footer'));
+        $this->assertFalse($items->contains('key', 'sidebar'));
+    }
+
     // -----------------------------------------------------------------------
     // Pluck / keyBy / groupBy
     // -----------------------------------------------------------------------