diff --git a/src/PHPStan/GetFlagValueFunctionDynamicReturnTypeExtension.php b/src/PHPStan/GetFlagValueFunctionDynamicReturnTypeExtension.php
index 57941e98..f367b1ed 100644
--- a/src/PHPStan/GetFlagValueFunctionDynamicReturnTypeExtension.php
+++ b/src/PHPStan/GetFlagValueFunctionDynamicReturnTypeExtension.php
@@ -61,22 +61,28 @@ public function getTypeFromFunctionCall(
 			$keyTypes               = $assocArgsConstantArray->getKeyTypes();
 			$valueTypes             = $assocArgsConstantArray->getValueTypes();
 			$resolvedValueType      = null;
+			$isOptionalKey          = false;
 
 			foreach ( $keyTypes as $index => $keyType ) {
 				$keyConstantStrings = $keyType->getConstantStrings();
 				if ( count( $keyConstantStrings ) === 1 && $keyConstantStrings[0]->getValue() === $flagValue ) {
 					$resolvedValueType = $valueTypes[ $index ];
+					$isOptionalKey     = $assocArgsConstantArray->isOptionalKey( $index );
 					break;
 				}
 			}
 
-			if ( null !== $resolvedValueType ) {
-				// Key definitely exists and has a resolved type.
-				return $resolvedValueType;
-			} else {
+			if ( null === $resolvedValueType ) {
 				// Key definitely does not exist in this constant array.
 				return $defaultType;
 			}
+
+			// get_flag_value() uses isset(), so a null value also falls back to the default.
+			if ( $resolvedValueType->isNull()->no() && ! $isOptionalKey ) {
+				return $resolvedValueType;
+			}
+
+			return TypeCombinator::union( TypeCombinator::removeNull( $resolvedValueType ), $defaultType );
 		}
 
 		// 4.b. $assoc_args is not a single ConstantArray (but $flagValue is known):
diff --git a/tests/data/get_flag_value.php b/tests/data/get_flag_value.php
index 9a6b39c2..f2046da9 100644
--- a/tests/data/get_flag_value.php
+++ b/tests/data/get_flag_value.php
@@ -60,3 +60,14 @@
 
 $value = get_flag_value( $assoc_args, $key2, 123 );
 assertType( "123|'bar'|true", $value );
+
+/**
+ * @param array{foo?: string, bar?: 'json'|'xml', baz: bool, qux: string|null} $assoc_args
+ */
+function test_shape_with_optional_keys( array $assoc_args ): void {
+	assertType( 'string|null', get_flag_value( $assoc_args, 'foo' ) );
+	assertType( "'json'|'xml'|null", get_flag_value( $assoc_args, 'bar' ) );
+	assertType( "'json'|'xml'|false", get_flag_value( $assoc_args, 'bar', false ) );
+	assertType( 'bool', get_flag_value( $assoc_args, 'baz', 123 ) );
+	assertType( '123|string', get_flag_value( $assoc_args, 'qux', 123 ) );
+}