diff --git a/array.c b/array.c
index 7b92bfe5b1..ec6f232662 100644
--- a/array.c
+++ b/array.c
@@ -1368,6 +1368,7 @@ ary_ensure_room_for_unshift(VALUE ary, int argc)
 	VALUE shared = ARY_SHARED(ary);
 	capa = RARRAY_LEN(shared);
 	if (ARY_SHARED_OCCUPIED(shared) && capa > new_len) {
+            rb_ary_modify_check(ary);
             head = RARRAY_CONST_PTR_TRANSIENT(ary);
             sharedp = RARRAY_CONST_PTR_TRANSIENT(shared);
 	    goto makeroom_if_need;
diff --git a/test/ruby/test_array.rb b/test/ruby/test_array.rb
index 94ffa73eea..8d48b86ea9 100644
--- a/test/ruby/test_array.rb
+++ b/test/ruby/test_array.rb
@@ -1934,6 +1934,17 @@ class TestArray < Test::Unit::TestCase
     assert_equal(@cls[@cls[1,2], nil, 'dog', 'cat'], a.unshift(@cls[1, 2]))
   end
 
+  def test_unshift_frozen
+    bug15952 = '[Bug #15952]'
+    assert_raise(FrozenError, bug15952) do
+      a = [1] * 100
+      b = a[4..-1]
+      a.replace([1])
+      b.freeze
+      b.unshift("a")
+    end
+  end
+
   def test_OR # '|'
     assert_equal(@cls[],  @cls[]  | @cls[])
     assert_equal(@cls[1], @cls[1] | @cls[])
diff --git a/version.h b/version.h
index 4059e381aa..f043008bce 100644
--- a/version.h
+++ b/version.h
@@ -1,6 +1,6 @@
 #define RUBY_VERSION "2.6.3"
 #define RUBY_RELEASE_DATE RUBY_RELEASE_YEAR_STR"-"RUBY_RELEASE_MONTH_STR"-"RUBY_RELEASE_DAY_STR
-#define RUBY_PATCHLEVEL 90
+#define RUBY_PATCHLEVEL 91
 
 #define RUBY_RELEASE_YEAR 2019
 #define RUBY_RELEASE_MONTH 8