diff --git a/rules-tests/TypeDeclaration/Rector/ClassMethod/StrictStringParamConcatRector/Fixture/skip_possible_array.php.inc b/rules-tests/TypeDeclaration/Rector/ClassMethod/StrictStringParamConcatRector/Fixture/skip_possible_array.php.inc
new file mode 100644
index 00000000000..d77736a6b58
--- /dev/null
+++ b/rules-tests/TypeDeclaration/Rector/ClassMethod/StrictStringParamConcatRector/Fixture/skip_possible_array.php.inc
@@ -0,0 +1,13 @@
+<?php
+
+final class SkipPossibleArray
+{
+    public function run($key): void
+    {
+        if (\is_array($key)) {
+            $dayKey = [$key[0], $key[1] . '_day'];
+        } else {
+            $dayKey = $key . '_day';
+        }
+    }
+}
diff --git a/rules-tests/TypeDeclaration/Rector/ClassMethod/StrictStringParamConcatRector/Fixture/skip_possible_array2.php.inc b/rules-tests/TypeDeclaration/Rector/ClassMethod/StrictStringParamConcatRector/Fixture/skip_possible_array2.php.inc
new file mode 100644
index 00000000000..de261efb24b
--- /dev/null
+++ b/rules-tests/TypeDeclaration/Rector/ClassMethod/StrictStringParamConcatRector/Fixture/skip_possible_array2.php.inc
@@ -0,0 +1,15 @@
+<?php
+
+final class SkipPossibleArray2
+{
+    public function run($key): void
+    {
+        if (\is_array($key)) {
+            $dayKey = [$key[0], $key[1] . '_day'];
+        } elseif (rand(0, 1)) {
+            $dayKey = $key . '_day';
+        } else {
+            $dayKey = null;
+        }
+    }
+}
diff --git a/rules/TypeDeclaration/Rector/ClassMethod/StrictStringParamConcatRector.php b/rules/TypeDeclaration/Rector/ClassMethod/StrictStringParamConcatRector.php
index 5aeb1f2a9cc..5a9794b127b 100644
--- a/rules/TypeDeclaration/Rector/ClassMethod/StrictStringParamConcatRector.php
+++ b/rules/TypeDeclaration/Rector/ClassMethod/StrictStringParamConcatRector.php
@@ -101,16 +101,22 @@ public function refactor(Node $node): ?Node
             }
 
             $nativeType = $this->nodeTypeResolver->getNativeType($variableConcattedFromParam);
+            if (! $nativeType instanceof MixedType) {
+                continue;
+            }
 
-            if ($nativeType instanceof MixedType && $nativeType->getSubtractedType() instanceof Type && TypeCombinator::containsNull(
-                $nativeType->getSubtractedType()
-            )) {
-                $param->type = new NullableType(new Identifier('string'));
-            } else {
+            $subtractedType = $nativeType->getSubtractedType();
+            if (! $subtractedType instanceof Type) {
                 $param->type = new Identifier('string');
+                $hasChanged = true;
+
+                continue;
             }
 
-            $hasChanged = true;
+            if (TypeCombinator::containsNull($subtractedType)) {
+                $param->type = new NullableType(new Identifier('string'));
+                $hasChanged = true;
+            }
         }
 
         if ($hasChanged) {