diff --git a/src/Mapper/Tree/Shell.php b/src/Mapper/Tree/Shell.php
index 150083cd..10831820 100644
--- a/src/Mapper/Tree/Shell.php
+++ b/src/Mapper/Tree/Shell.php
@@ -80,6 +80,7 @@ public function withType(Type $newType): self
     {
         $clone = clone $this;
         $clone->type = $newType;
+        $clone->value = self::castCompatibleValue($newType, $this->value);
 
         return $clone;
     }
@@ -91,17 +92,9 @@ public function type(): Type
 
     public function withValue(mixed $value): self
     {
-        // When the value is an integer and the type is a float, the value is
-        // cast to float, to follow the rule of PHP regarding acceptance of an
-        // integer value in a float type. Note that PHPStan/Psalm analysis
-        // applies the same rule.
-        if ($this->type instanceof FloatType && is_int($value)) {
-            $value = (float)$value;
-        }
-
         $clone = clone $this;
         $clone->hasValue = true;
-        $clone->value = $value;
+        $clone->value = self::castCompatibleValue($clone->type, $value);
 
         return $clone;
     }
@@ -173,4 +166,17 @@ public function path(): string
 
         return implode('.', $path);
     }
+
+    private static function castCompatibleValue(Type $type, mixed $value): mixed
+    {
+        // When the value is an integer and the type is a float, the value is
+        // cast to float, to follow the rule of PHP regarding acceptance of an
+        // integer value in a float type. Note that PHPStan/Psalm analysis
+        // applies the same rule.
+        if ($type instanceof FloatType && is_int($value)) {
+            return (float)$value;
+        }
+
+        return $value;
+    }
 }
diff --git a/tests/Integration/Mapping/UnionMappingTest.php b/tests/Integration/Mapping/UnionMappingTest.php
index 8612d507..e840a98c 100644
--- a/tests/Integration/Mapping/UnionMappingTest.php
+++ b/tests/Integration/Mapping/UnionMappingTest.php
@@ -64,6 +64,15 @@ public static function union_mapping_works_properly_data_provider(): iterable
             'assertion' => fn (mixed $result) => self::assertNull($result),
         ];
 
+        yield 'nullable float with integer value' => [
+            'type' => 'float|null',
+            'source' => 42,
+            'assertion' => function (mixed $result) {
+                self::assertIsFloat($result);
+                self::assertEquals(42.0, $result);
+            },
+        ];
+
         yield 'string or list of string, with string' => [
             'type' => 'string|list<string>',
             'source' => 'foo',