diff --git a/src/Rules/SimplifyExpectNotRector.php b/src/Rules/SimplifyExpectNotRector.php
index bfb441d..e935493 100644
--- a/src/Rules/SimplifyExpectNotRector.php
+++ b/src/Rules/SimplifyExpectNotRector.php
@@ -100,6 +100,10 @@ public function refactor(Node $node): ?Node
         }
 
         if (isset(self::FLIPPABLE_MATCHERS[$finalMethodName])) {
+            if (! $this->getType($arg->value->expr)->isBoolean()->yes()) {
+                return null;
+            }
+
             $expectCall->args[0] = $this->nodeFactory->createArg($arg->value->expr);
 
             $this->flipFinalMatcher($node, self::FLIPPABLE_MATCHERS[$finalMethodName]);
diff --git a/src/Rules/ToBeTrueNotFalseRector.php b/src/Rules/ToBeTrueNotFalseRector.php
index d19ea43..9fb53a7 100644
--- a/src/Rules/ToBeTrueNotFalseRector.php
+++ b/src/Rules/ToBeTrueNotFalseRector.php
@@ -81,7 +81,7 @@ public function refactor(Node $node): ?Node
             return null;
         }
 
-        if ($this->getType($expectArgument)->isBoolean()->no()) {
+        if (! $this->getType($expectArgument)->isBoolean()->yes()) {
             return null;
         }
 
diff --git a/tests/Rules/SimplifyExpectNotRector/Fixture/simplify_expect_not.php.inc b/tests/Rules/SimplifyExpectNotRector/Fixture/simplify_expect_not.php.inc
index 3178e6c..693dea0 100644
--- a/tests/Rules/SimplifyExpectNotRector/Fixture/simplify_expect_not.php.inc
+++ b/tests/Rules/SimplifyExpectNotRector/Fixture/simplify_expect_not.php.inc
@@ -1,5 +1,8 @@
 <?php
 
+$condition = random_int(0, 1) === 1;
+$value = random_int(0, 1) === 1;
+
 expect(!$condition)->toBeTrue();
 expect(!$value)->toBeFalse();
 expect(!empty($array))->toBeTrue();
@@ -9,6 +12,9 @@ expect(!is_null($variable))->toBeTrue();
 -----
 <?php
 
+$condition = random_int(0, 1) === 1;
+$value = random_int(0, 1) === 1;
+
 expect($condition)->toBeFalse();
 expect($value)->toBeTrue();
 expect(empty($array))->toBeFalse();
diff --git a/tests/Rules/SimplifyExpectNotRector/Fixture/skip_non_bool_type.php.inc b/tests/Rules/SimplifyExpectNotRector/Fixture/skip_non_bool_type.php.inc
new file mode 100644
index 0000000..fa1885c
--- /dev/null
+++ b/tests/Rules/SimplifyExpectNotRector/Fixture/skip_non_bool_type.php.inc
@@ -0,0 +1,8 @@
+<?php
+
+$haystack = (string) random_int(0, 9);
+
+expect(!random_int(0, 9))->toBeTrue();
+expect(!mb_strstr($haystack, 'needle'))->toBeTrue();
+
+?>
diff --git a/tests/Rules/ToBeTrueNotFalseRector/Fixture/skip_union_string_false.php.inc b/tests/Rules/ToBeTrueNotFalseRector/Fixture/skip_union_string_false.php.inc
new file mode 100644
index 0000000..f46a489
--- /dev/null
+++ b/tests/Rules/ToBeTrueNotFalseRector/Fixture/skip_union_string_false.php.inc
@@ -0,0 +1,7 @@
+<?php
+
+test('skips a string|false union subject', function (string $haystack): void {
+    expect(mb_strstr($haystack, 'needle'))->not->toBeFalse();
+});
+
+?>