diff --git a/tests/unit/Converter/Anonymizer/AnonymizeDateTest.php b/tests/unit/Converter/Anonymizer/AnonymizeDateTest.php
index 3a7ad29f..0a3c11be 100644
--- a/tests/unit/Converter/Anonymizer/AnonymizeDateTest.php
+++ b/tests/unit/Converter/Anonymizer/AnonymizeDateTest.php
@@ -24,6 +24,7 @@ public function testConverter(): void
 
         $date = '1990-12-31';
         $value = $converter->convert($date);
+        $this->assertIsString($value);
         $this->assertDateIsAnonymized($value, $date, 'Y-m-d');
     }
 
@@ -37,6 +38,7 @@ public function testFormatParameter(): void
 
         $date = '31/12/1990';
         $value = $converter->convert($date);
+        $this->assertIsString($value);
         $this->assertDateIsAnonymized($value, $date, $format);
     }
 
diff --git a/tests/unit/Converter/Anonymizer/AnonymizeDateTimeTest.php b/tests/unit/Converter/Anonymizer/AnonymizeDateTimeTest.php
index 91957185..7b58b7a9 100644
--- a/tests/unit/Converter/Anonymizer/AnonymizeDateTimeTest.php
+++ b/tests/unit/Converter/Anonymizer/AnonymizeDateTimeTest.php
@@ -19,6 +19,7 @@ public function testConverter(): void
 
         $date = '1990-12-31 12:05:41';
         $value = $converter->convert($date);
+        $this->assertIsString($value);
         $this->assertDateIsAnonymized($value, $date, 'Y-m-d H:i:s');
     }
 
@@ -32,6 +33,7 @@ public function testFormatParameter(): void
 
         $date = '31/12/1990 12:05:41';
         $value = $converter->convert($date);
+        $this->assertIsString($value);
         $this->assertDateIsAnonymized($value, $date, $format);
     }
 
diff --git a/tests/unit/Converter/Generator/RandomDateTest.php b/tests/unit/Converter/Generator/RandomDateTest.php
index c622791e..04a6156f 100644
--- a/tests/unit/Converter/Generator/RandomDateTest.php
+++ b/tests/unit/Converter/Generator/RandomDateTest.php
@@ -23,6 +23,7 @@ public function testConverter(): void
 
         $date = '1990-12-31';
         $value = $converter->convert($date);
+        $this->assertIsString($value);
         $this->assertDateIsRandomized($value, $date, 'Y-m-d');
     }
 
@@ -36,6 +37,7 @@ public function testFormatParameter(): void
 
         $date = '31/12/1990';
         $value = $converter->convert($date);
+        $this->assertIsString($value);
         $this->assertDateIsRandomized($value, $date, $format);
     }
 
@@ -51,6 +53,7 @@ public function testYearParameters(): void
 
         $date = '1990-12-31';
         $value = $converter->convert($date);
+        $this->assertIsString($value);
         $this->assertDateIsRandomized($value, $date, 'Y-m-d');
     }
 
@@ -66,6 +69,7 @@ public function testNullYears(): void
 
         $date = '1990-12-31';
         $value = $converter->convert($date);
+        $this->assertIsString($value);
 
         $currentYear = (new DateTime())->format('Y');
         $randomizedYear = (new DateTime($value))->format('Y');
diff --git a/tests/unit/Converter/Generator/RandomDateTimeTest.php b/tests/unit/Converter/Generator/RandomDateTimeTest.php
index a4e5f499..4d3b2acb 100644
--- a/tests/unit/Converter/Generator/RandomDateTimeTest.php
+++ b/tests/unit/Converter/Generator/RandomDateTimeTest.php
@@ -22,6 +22,7 @@ public function testConverter(): void
 
         $date = '1990-12-31 12:05:41';
         $value = $converter->convert($date);
+        $this->assertIsString($value);
         $this->assertDateIsRandomized($value, $date, 'Y-m-d H:i:s');
     }
 
@@ -35,6 +36,7 @@ public function testFormatParameter(): void
 
         $date = '31/12/1990 12:05:41';
         $randomizedDate = $converter->convert($date);
+        $this->assertIsString($randomizedDate);
         $this->assertDateIsRandomized($randomizedDate, $date, $format);
     }
 
@@ -50,6 +52,7 @@ public function testYearParameters(): void
 
         $date = '1990-12-31 12:05:41';
         $randomizedDate = $converter->convert($date);
+        $this->assertIsString($randomizedDate);
         $this->assertDateIsRandomized($randomizedDate, $date, 'Y-m-d H:i:s');
     }
 
@@ -65,6 +68,7 @@ public function testNullYears(): void
 
         $date = '1990-12-31 12:05:41';
         $randomizedDate = $converter->convert($date);
+        $this->assertIsString($randomizedDate);
 
         $currentYear = (new DateTime())->format('Y');
         $randomizedYear = (new DateTime($randomizedDate))->format('Y');
diff --git a/tests/unit/Converter/Generator/RandomEmailTest.php b/tests/unit/Converter/Generator/RandomEmailTest.php
index 089769fb..9ac6edfa 100644
--- a/tests/unit/Converter/Generator/RandomEmailTest.php
+++ b/tests/unit/Converter/Generator/RandomEmailTest.php
@@ -21,6 +21,7 @@ public function testConverter(): void
         $this->assertNotNull($value);
 
         $value = $converter->convert('user1@gmail.com');
+        $this->assertIsString($value);
         $this->assertStringNotContainsString('user1', $value);
         $this->assertStringNotContainsString('@gmail.com', $value);
         $this->assertStringEndsWith('@example.org', $value);
@@ -38,6 +39,7 @@ public function testCustomLength(): void
         ]);
 
         $value = $converter->convert('user1@example.org');
+        $this->assertIsString($value);
         $this->assertStringEndsWith('@example.org', $value);
 
         $parts = explode('@', $value);
diff --git a/tests/unit/Converter/Generator/RandomTextTest.php b/tests/unit/Converter/Generator/RandomTextTest.php
index af9ce1ef..facdf44f 100644
--- a/tests/unit/Converter/Generator/RandomTextTest.php
+++ b/tests/unit/Converter/Generator/RandomTextTest.php
@@ -21,6 +21,7 @@ public function testConverter(): void
         $this->assertNotNull($value);
 
         $value = $converter->convert('user1');
+        $this->assertIsString($value);
         $this->assertStringNotContainsString('user1', $value);
         $this->assertGreaterThanOrEqual(3, strlen($value));
     }
@@ -37,6 +38,7 @@ public function testCustomLength(): void
         ]);
 
         $value = $converter->convert('user1');
+        $this->assertIsString($value);
         $this->assertSame(20, strlen($value));
     }
 
diff --git a/tests/unit/Converter/Proxy/JsonDataTest.php b/tests/unit/Converter/Proxy/JsonDataTest.php
index 822ad1a8..44411fb4 100644
--- a/tests/unit/Converter/Proxy/JsonDataTest.php
+++ b/tests/unit/Converter/Proxy/JsonDataTest.php
@@ -29,6 +29,7 @@ public function testConverter(): void
         $this->assertNull($value);
 
         $value = $converter->convert($this->getJsonData());
+        $this->assertIsString($value);
         $this->assertJson($this->getExpectedData(), $value);
     }
 
diff --git a/tests/unit/Converter/Randomizer/RandomizeEmailTest.php b/tests/unit/Converter/Randomizer/RandomizeEmailTest.php
index 589ac525..b37b603a 100644
--- a/tests/unit/Converter/Randomizer/RandomizeEmailTest.php
+++ b/tests/unit/Converter/Randomizer/RandomizeEmailTest.php
@@ -21,6 +21,7 @@ public function testConverter(): void
         $this->assertSame('', $value);
 
         $value = $converter->convert('user1@gmail.com');
+        $this->assertIsString($value);
         $this->assertStringNotContainsString('user1', $value);
         $this->assertStringNotContainsString('@gmail.com', $value);
         $this->assertStringEndsWith('@example.org', $value);
@@ -37,6 +38,7 @@ public function testCustomLength(): void
         ]);
 
         $value = $converter->convert('user1@example.org');
+        $this->assertIsString($value);
         $this->assertStringEndsWith('@example.org', $value);
 
         $parts = explode('@', $value);
diff --git a/tests/unit/Converter/Randomizer/RandomizeNumberTest.php b/tests/unit/Converter/Randomizer/RandomizeNumberTest.php
index 21af41ea..016da44e 100644
--- a/tests/unit/Converter/Randomizer/RandomizeNumberTest.php
+++ b/tests/unit/Converter/Randomizer/RandomizeNumberTest.php
@@ -20,12 +20,13 @@ public function testConverter(): void
         $this->assertSame('', $value);
 
         $value = $converter->convert('+33601010101');
+        $this->assertIsString($value);
         $this->assertStringStartsWith('+', $value);
         $this->assertSame(12, strlen($value));
 
         // Assert that the part without the "+" is still a numeric value
         $valueWithoutPlus = substr($value, 1);
-        $this->assertTrue(is_numeric($valueWithoutPlus));
+        $this->assertIsNumeric($valueWithoutPlus);
         $this->assertNotSame('33601010101', $valueWithoutPlus);
     }
 }
diff --git a/tests/unit/Converter/Randomizer/RandomizeTextTest.php b/tests/unit/Converter/Randomizer/RandomizeTextTest.php
index a2fe487d..1a7665e0 100644
--- a/tests/unit/Converter/Randomizer/RandomizeTextTest.php
+++ b/tests/unit/Converter/Randomizer/RandomizeTextTest.php
@@ -21,6 +21,7 @@ public function testConverter(): void
         $this->assertSame('', $value);
 
         $value = $converter->convert('user1');
+        $this->assertIsString($value);
         $this->assertStringNotContainsString('user1', $value);
     }
 
@@ -32,6 +33,7 @@ public function testCustomLength(): void
         $converter = $this->createConverter(RandomizeText::class, ['min_length' => 10]);
 
         $value = $converter->convert('user1');
+        $this->assertIsString($value);
         $this->assertSame(10, strlen($value));
     }
 
diff --git a/tests/unit/Converter/Transformer/HashTest.php b/tests/unit/Converter/Transformer/HashTest.php
index 75f8568b..7ffe7749 100644
--- a/tests/unit/Converter/Transformer/HashTest.php
+++ b/tests/unit/Converter/Transformer/HashTest.php
@@ -22,6 +22,7 @@ public function testConverter(): void
         $this->assertSame('', $value);
 
         $value = $converter->convert('user1');
+        $this->assertIsString($value);
         $this->assertSame(40, strlen($value));
     }
 
@@ -33,6 +34,7 @@ public function testCustomAlgorithm(): void
         $converter = $this->createConverter(Hash::class, ['algorithm' => 'sha256']);
 
         $value = $converter->convert('user1');
+        $this->assertIsString($value);
         $this->assertSame(64, strlen($value));
     }