diff --git a/src/Psalm/Internal/Type/Comparator/UnionTypeComparator.php b/src/Psalm/Internal/Type/Comparator/UnionTypeComparator.php
index a3fd5e843f2..6ed8f7ea14c 100644
--- a/src/Psalm/Internal/Type/Comparator/UnionTypeComparator.php
+++ b/src/Psalm/Internal/Type/Comparator/UnionTypeComparator.php
@@ -22,6 +22,9 @@
 use function array_pop;
 use function array_push;
 use function array_reverse;
+use function count;
+
+use const PHP_INT_MAX;
 
 /**
  * @internal
@@ -136,7 +139,10 @@ public static function isContainedBy(
                 }
 
                 // if params are specified
-                if ($container_type_part instanceof TCallable && $container_type_part->params !== null && $input_type_part instanceof TCallable) {
+                if ($container_type_part instanceof TCallable
+                    && is_array($container_type_part->params)
+                    && $input_type_part instanceof TCallable
+                ) {
                     $container_required_param_count = 0;
                     foreach ($container_type_part->params as $index => $container_param) {
                         if ($container_param->is_optional === false) {
@@ -145,14 +151,22 @@ public static function isContainedBy(
                     }
 
                     $input_required_param_count = 0;
-                    foreach ($input_type_part->params as $index => $input_param) {
-                        if ($input_param->is_optional === false) {
-                            $input_required_param_count = $index + 1;
+                    if (!is_array($input_type_part->params)) {
+                        // it's not declared, there can be an arbitrary number of params
+                        $input_all_param_count = PHP_INT_MAX;
+                    } else {
+                        $input_all_param_count = count($input_type_part->params);
+                        foreach ($input_type_part->params as $index => $input_param) {
+                            if ($input_param->is_optional === false) {
+                                $input_required_param_count = $index + 1;
+                            }
                         }
                     }
 
                     // too few or too many non-optional params provided in callback
-                    if ($container_required_param_count > count($input_type_part->params) || count($container_type_part->params) < $input_required_param_count) {
+                    if ($container_required_param_count > $input_all_param_count
+                        || count($container_type_part->params) < $input_required_param_count
+                    ) {
                         return false;
                     }
                 }