diff --git a/app/Helpers/migration_helper.php b/app/Helpers/migration_helper.php
index 0e86143c6..d7da83922 100644
--- a/app/Helpers/migration_helper.php
+++ b/app/Helpers/migration_helper.php
@@ -1,5 +1,7 @@
 <?php
 
+use CodeIgniter\Database\BaseConnection;
+use CodeIgniter\Database\Exceptions\DatabaseException;
 use Config\Database;
 
 /**
@@ -118,13 +120,20 @@ function createPrimaryKey(string $table, string $index): void {
 /**
  * Drops all foreign key constraints that reference the provided table and column.
  *
+ * A failed metadata query is treated as an error (thrown) rather than mistaken
+ * for "no constraints found", so callers never proceed with schema changes on a
+ * wrong assumption.
+ *
  * @param string $table
  * @param string $column
+ * @param BaseConnection|null $db Connection to use (defaults to the default connection).
+ *
  * @return array containing the deleted constraints in case they need to be recreated after.
+ *
+ * @throws DatabaseException If the metadata query fails.
  */
-
-function dropAllForeignKeyConstraints(string $table, string $column): array {
-    $db = Database::connect();
+function dropAllForeignKeyConstraints(string $table, string $column, ?BaseConnection $db = null): array {
+    $db ??= Database::connect();
     $result = $db->query("
             SELECT DISTINCT
                 kcu.CONSTRAINT_NAME,
@@ -144,6 +153,23 @@ function dropAllForeignKeyConstraints(string $table, string $column): array {
                 AND rc.CONSTRAINT_NAME IS NOT NULL
         ");
 
+    if ($result === false) {
+        $detail = 'unknown error';
+        try {
+            $error = $db->error();
+            if (is_array($error) && !empty($error['message'])) {
+                $detail = (string) $error['message'];
+            }
+        } catch (\Throwable $e) {
+            // A disconnected or mocked connection may not support error(); keep the default.
+        }
+
+        throw new DatabaseException(sprintf(
+            'Failed to look up foreign key constraints referencing table "%s" column "%s": %s',
+            $table, $column, $detail
+        ));
+    }
+
     $deletedConstraints = [];
 
     foreach ($result->getResultArray() as $constraint) {
@@ -187,22 +213,72 @@ function deleteIndex(string $table, string $index): void {
  *
  * @param string $table
  * @param string $index
+ * @param BaseConnection|null $db Connection to use (defaults to the default connection).
+ *
  * @return bool
+ *
+ * @throws DatabaseException If the metadata query fails.
  */
-function indexExists(string $table, string $index): bool {
-    $db = Database::connect();
+function indexExists(string $table, string $index, ?BaseConnection $db = null): bool {
+    $db ??= Database::connect();
     $result = $db->query('SELECT COUNT(*) FROM information_schema.statistics WHERE table_schema = DATABASE() AND table_name = \'' . $db->getPrefix() . "$table' AND index_name = '$index'");
+
+    if ($result === false) {
+        $detail = 'unknown error';
+        try {
+            $error = $db->error();
+            if (is_array($error) && !empty($error['message'])) {
+                $detail = (string) $error['message'];
+            }
+        } catch (\Throwable $e) {
+            // A disconnected or mocked connection may not support error(); keep the default.
+        }
+
+        throw new DatabaseException(sprintf(
+            'Failed to check whether index "%s" exists on table "%s": %s',
+            $index, $table, $detail
+        ));
+    }
+
     $row_array = $result->getRowArray();
 
-    return $row_array && $row_array['COUNT(*)'] > 0;
+    return $row_array !== null && $row_array['COUNT(*)'] > 0;
 }
 
-function primaryKeyExists(string $table): bool {
-    $db = Database::connect();
+/**
+ * Checks if the specified table has a primary key.
+ *
+ * @param string $table
+ * @param BaseConnection|null $db Connection to use (defaults to the default connection).
+ *
+ * @return bool
+ *
+ * @throws DatabaseException If the metadata query fails.
+ */
+function primaryKeyExists(string $table, ?BaseConnection $db = null): bool {
+    $db ??= Database::connect();
     $result = $db->query('SELECT COUNT(*) FROM information_schema.table_constraints WHERE table_schema = DATABASE() AND table_name = \'' . $db->getPrefix() . "$table' AND constraint_type = 'PRIMARY KEY'");
+
+    if ($result === false) {
+        $detail = 'unknown error';
+        try {
+            $error = $db->error();
+            if (is_array($error) && !empty($error['message'])) {
+                $detail = (string) $error['message'];
+            }
+        } catch (\Throwable $e) {
+            // A disconnected or mocked connection may not support error(); keep the default.
+        }
+
+        throw new DatabaseException(sprintf(
+            'Failed to check whether table "%s" has a primary key: %s',
+            $table, $detail
+        ));
+    }
+
     $row_array = $result->getRowArray();
 
-    return $row_array && $row_array['COUNT(*)'] > 0;
+    return $row_array !== null && $row_array['COUNT(*)'] > 0;
 }
 
 function recreateForeignKeyConstraints(array $constraints): void {
diff --git a/app/Models/Reports/Inventory_low.php b/app/Models/Reports/Inventory_low.php
index 65f7a1b14..7dfb16d8a 100644
--- a/app/Models/Reports/Inventory_low.php
+++ b/app/Models/Reports/Inventory_low.php
@@ -47,7 +47,7 @@ class Inventory_low extends Report
             AND stock_locations.deleted = 0
             ORDER BY items.name");
 
-        return $query->getResultArray() ?: [];
+        return $query ? $query->getResultArray() : [];
     }
 
     /**
diff --git a/app/Models/Sale.php b/app/Models/Sale.php
index e1a0da097..45f84ab49 100644
--- a/app/Models/Sale.php
+++ b/app/Models/Sale.php
@@ -1193,7 +1193,7 @@ class Sale extends Model
                 . $this->db->prefixTable('sales') . ' where sale_status = ' . SUSPENDED . ' AND customer_id = ' . $customer_id);
         }
 
-        return $query->getResultArray() ?: [];
+        return $query ? $query->getResultArray() : [];
     }
 
     /**
diff --git a/app/Models/Tax.php b/app/Models/Tax.php
index 61e6e9361..dfa7783e3 100644
--- a/app/Models/Tax.php
+++ b/app/Models/Tax.php
@@ -127,7 +127,7 @@ class Tax extends Model
 
         $query = $this->db->query($sql);
 
-        return $query->getResultArray() ?: [];
+        return $query ? $query->getResultArray() : [];
     }
 
     /**
diff --git a/tests/Models/Reports/Inventory_lowTest.php b/tests/Models/Reports/Inventory_lowTest.php
new file mode 100644
index 000000000..92840294f
--- /dev/null
+++ b/tests/Models/Reports/Inventory_lowTest.php
@@ -0,0 +1,34 @@
+<?php
+
+namespace Tests\Models\Reports;
+
+use App\Models\Reports\Inventory_low;
+use CodeIgniter\Test\CIUnitTestCase;
+use CodeIgniter\Test\DatabaseTestTrait;
+use Tests\Support\FailedQueryConnectionTrait;
+
+/**
+ * Regression test for Issue #3634: Inventory_low::getData() must degrade to an
+ * empty result (not throw "Call to member function ... on false") when the
+ * query fails.
+ */
+class Inventory_lowTest extends CIUnitTestCase
+{
+    use DatabaseTestTrait;
+    use FailedQueryConnectionTrait;
+
+    protected $migrate     = true;
+    protected $migrateOnce = true;
+    protected $refresh     = false;
+    protected $namespace   = null;
+
+    /**
+     * Inventory_low::getData() must return an empty array, not throw, when the query fails.
+     */
+    public function testInventoryLowGetData_ReturnsEmptyArrayWhenQueryFails(): void
+    {
+        $report = $this->modelWithFailingQuery(Inventory_low::class);
+
+        $this->assertSame([], $report->getData([]));
+    }
+}
diff --git a/tests/Models/SaleTest.php b/tests/Models/SaleTest.php
index aff3b79c5..e6372d754 100644
--- a/tests/Models/SaleTest.php
+++ b/tests/Models/SaleTest.php
@@ -7,6 +7,7 @@ use CodeIgniter\Database\Config;
 use CodeIgniter\Test\CIUnitTestCase;
 use CodeIgniter\Test\DatabaseTestTrait;
 use Tests\Support\EmployeeFixtureTrait;
+use Tests\Support\FailedQueryConnectionTrait;
 use Tests\Support\ItemFixtureTrait;
 
 /**
@@ -18,6 +19,7 @@ class SaleTest extends CIUnitTestCase
 {
     use DatabaseTestTrait;
     use EmployeeFixtureTrait;
+    use FailedQueryConnectionTrait;
     use ItemFixtureTrait;
 
     protected $migrate     = true;
@@ -434,4 +436,14 @@ class SaleTest extends CIUnitTestCase
         $this->assertTrue($deleteResult);
         $this->assertEqualsWithDelta(10.0, $this->getItemQuantity($itemId, self::LOCATION_ID), 0.001);
     }
+
+    public function testGetAllSuspended_ReturnsEmptyArrayWhenQueryFails(): void
+    {
+        $model = $this->modelWithFailingQuery(Sale::class);
+
+        // Exercises both the NEW_ENTRY branch and the customer-id branch.
+        $this->assertSame([], $model->get_all_suspended());
+        $this->assertSame([], $model->get_all_suspended(NEW_ENTRY));
+        $this->assertSame([], $model->get_all_suspended(123));
+    }
 }
diff --git a/tests/Models/TaxTest.php b/tests/Models/TaxTest.php
new file mode 100644
index 000000000..9075cf70c
--- /dev/null
+++ b/tests/Models/TaxTest.php
@@ -0,0 +1,27 @@
+<?php
+
+namespace Tests\Models;
+
+use App\Models\Tax;
+use CodeIgniter\Test\CIUnitTestCase;
+use CodeIgniter\Test\DatabaseTestTrait;
+use Tests\Support\FailedQueryConnectionTrait;
+
+class TaxTest extends CIUnitTestCase
+{
+    use DatabaseTestTrait;
+    use FailedQueryConnectionTrait;
+
+    protected $migrate     = true;
+    protected $migrateOnce = true;
+    protected $seedOnce    = true;
+    protected $refresh     = false;
+    protected $namespace   = null;
+
+    public function testGetTaxes_ReturnsEmptyArrayWhenQueryFails(): void
+    {
+        $model = $this->modelWithFailingQuery(Tax::class);
+
+        $this->assertSame([], $model->get_taxes(1, 1));
+    }
+}
diff --git a/tests/Support/FailedQueryConnectionTrait.php b/tests/Support/FailedQueryConnectionTrait.php
new file mode 100644
index 000000000..6906e703e
--- /dev/null
+++ b/tests/Support/FailedQueryConnectionTrait.php
@@ -0,0 +1,41 @@
+<?php
+
+namespace Tests\Support;
+
+use CodeIgniter\BaseModel;
+use CodeIgniter\Database\BaseConnection;
+
+trait FailedQueryConnectionTrait
+{
+    protected function failingConnection(): BaseConnection
+    {
+        $driverClass = get_class(\Config\Database::connect($this->DBGroup));
+
+        $db = $this->getMockBuilder($driverClass)
+            ->disableOriginalConstructor()
+            ->onlyMethods(['query', 'error'])
+            ->getMock();
+        $db->method('query')->willReturn(false);
+        $db->method('error')->willReturn(['code' => 1, 'message' => 'simulated query failure']);
+
+        return $db;
+    }
+
+    /**
+     * @template T of object
+     *
+     * @param class-string<T> $class
+     *
+     * @return T
+     */
+    protected function modelWithFailingQuery(string $class)
+    {
+        $model = new $class();
+
+        $prop = new \ReflectionProperty(BaseModel::class, 'db');
+        $prop->setAccessible(true);
+        $prop->setValue($model, $this->failingConnection());
+
+        return $model;
+    }
+}
diff --git a/tests/helpers/migration_helperTest.php b/tests/helpers/migration_helperTest.php
new file mode 100644
index 000000000..5fe2e5555
--- /dev/null
+++ b/tests/helpers/migration_helperTest.php
@@ -0,0 +1,53 @@
+<?php
+
+namespace Tests\helpers;
+
+use CodeIgniter\Database\Exceptions\DatabaseException;
+use CodeIgniter\Test\CIUnitTestCase;
+use CodeIgniter\Test\DatabaseTestTrait;
+use Tests\Support\FailedQueryConnectionTrait;
+
+class migration_helperTest extends CIUnitTestCase
+{
+    use DatabaseTestTrait;
+    use FailedQueryConnectionTrait;
+
+    protected $migrate     = true;
+    protected $migrateOnce = true;
+    protected $refresh     = false;
+    protected $namespace   = null;
+
+    public function testIndexExists_ThrowsWhenQueryFails(): void
+    {
+        $this->requireMigrationHelper();
+        $this->expectException(DatabaseException::class);
+
+        indexExists('some_table', 'some_index', $this->failingConnection());
+    }
+
+    public function testPrimaryKeyExists_ThrowsWhenQueryFails(): void
+    {
+        $this->requireMigrationHelper();
+        $this->expectException(DatabaseException::class);
+
+        primaryKeyExists('some_table', $this->failingConnection());
+    }
+
+    public function testDropAllForeignKeyConstraints_ThrowsWhenQueryFails(): void
+    {
+        $this->requireMigrationHelper();
+        $this->expectException(DatabaseException::class);
+
+        dropAllForeignKeyConstraints('some_table', 'some_column', $this->failingConnection());
+    }
+
+    /**
+     * Ensure the migration helper functions are loaded (they are global).
+     */
+    private function requireMigrationHelper(): void
+    {
+        if (! function_exists('indexExists')) {
+            require APPPATH . 'Helpers/migration_helper.php';
+        }
+    }
+}