diff --git a/src/Analyser/TypeSpecifier.php b/src/Analyser/TypeSpecifier.php
index 4f3e7f9c43..f50bacc339 100644
--- a/src/Analyser/TypeSpecifier.php
+++ b/src/Analyser/TypeSpecifier.php
@@ -1964,11 +1964,15 @@ private function processBooleanSureConditionalTypes(Scope $scope, SpecifiedTypes
 					continue;
 				}
 
-				$holder = new ConditionalExpressionHolder(
-					$conditions,
-					ExpressionTypeHolder::createYes($expr, TypeCombinator::intersect($scope->getType($expr), $type)),
-				);
-				$holders[$exprString][$holder->getKey()] = $holder;
+				$resultTypeHolder = ExpressionTypeHolder::createYes($expr, TypeCombinator::intersect($scope->getType($expr), $type));
+
+				foreach ($this->expandUnionConditions($conditions) as $expandedConditions) {
+					$holder = new ConditionalExpressionHolder(
+						$expandedConditions,
+						$resultTypeHolder,
+					);
+					$holders[$exprString][$holder->getKey()] = $holder;
+				}
 			}
 
 			return $holders;
@@ -2085,11 +2089,15 @@ private function processBooleanNotSureConditionalTypes(Scope $scope, SpecifiedTy
 					continue;
 				}
 
-				$holder = new ConditionalExpressionHolder(
-					$conditions,
-					ExpressionTypeHolder::createYes($expr, TypeCombinator::remove($scope->getType($expr), $type)),
-				);
-				$holders[$exprString][$holder->getKey()] = $holder;
+				$resultTypeHolder = ExpressionTypeHolder::createYes($expr, TypeCombinator::remove($scope->getType($expr), $type));
+
+				foreach ($this->expandUnionConditions($conditions) as $expandedConditions) {
+					$holder = new ConditionalExpressionHolder(
+						$expandedConditions,
+						$resultTypeHolder,
+					);
+					$holders[$exprString][$holder->getKey()] = $holder;
+				}
 			}
 
 			return $holders;
@@ -2098,6 +2106,29 @@ private function processBooleanNotSureConditionalTypes(Scope $scope, SpecifiedTy
 		return [];
 	}
 
+	/**
+	 * @param array<string, ExpressionTypeHolder> $conditions
+	 * @return list<array<string, ExpressionTypeHolder>>
+	 */
+	private function expandUnionConditions(array $conditions): array
+	{
+		$result = [$conditions];
+		foreach ($conditions as $exprString => $typeHolder) {
+			$type = $typeHolder->getType();
+			if (!$type instanceof UnionType) {
+				continue;
+			}
+
+			foreach ($type->getTypes() as $innerType) {
+				$expanded = $conditions;
+				$expanded[$exprString] = ExpressionTypeHolder::createYes($typeHolder->getExpr(), $innerType);
+				$result[] = $expanded;
+			}
+		}
+
+		return $result;
+	}
+
 	/**
 	 * @return array{Expr, ConstantScalarType, Type}|null
 	 */
diff --git a/tests/PHPStan/Analyser/nsrt/bug-13591.php b/tests/PHPStan/Analyser/nsrt/bug-13591.php
new file mode 100644
index 0000000000..fea4eafdb0
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-13591.php
@@ -0,0 +1,16 @@
+<?php
+
+namespace Bug13591;
+
+use function PHPStan\Testing\assertType;
+
+function processAction(string $action, ?int $hotelId): void
+{
+	if ($hotelId === null && ($action === 'get_rooms' || $action === 'update')) {
+		throw new \InvalidArgumentException('Hotel ID is required');
+	}
+
+	if ($action === 'get_rooms') {
+		assertType('int', $hotelId);
+	}
+}