diff --git a/NEWS.md b/NEWS.md
index 27c535919a..2b96b56076 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -11,6 +11,14 @@ Note that each entry is kept to a minimum, see links for details.
 
 Note: We're only listing outstanding class updates.
 
+* Array
+
+    * `Array#pack` now raises ArgumentError for unknown directives. [[Bug #19150]]
+
+* String
+
+    * `String#unpack` now raises ArgumentError for unknown directives. [[Bug #19150]]
+
 ## Stdlib updates
 
 *   The following default gems are updated.
@@ -41,3 +49,5 @@ See GitHub releases like [GitHub Releases of Logger](https://github.com/ruby/log
 ## Implementation improvements
 
 ## JIT
+
+[Bug #19150]:         https://bugs.ruby-lang.org/issues/19150
diff --git a/pack.c b/pack.c
index 3bdae01362..f1f474759a 100644
--- a/pack.c
+++ b/pack.c
@@ -167,7 +167,7 @@ unknown_directive(const char *mode, char type, VALUE fmt)
         snprintf(unknown, sizeof(unknown), "\\x%.2x", type & 0xff);
     }
     fmt = rb_str_quote_unprintable(fmt);
-    rb_warn("unknown %s directive '%s' in '%"PRIsVALUE"'",
+    rb_raise(rb_eArgError, "unknown %s directive '%s' in '%"PRIsVALUE"'",
             mode, unknown, fmt);
 }
 
diff --git a/test/ruby/test_pack.rb b/test/ruby/test_pack.rb
index 0ee36857ba..27573ef457 100644
--- a/test/ruby/test_pack.rb
+++ b/test/ruby/test_pack.rb
@@ -777,32 +777,32 @@ EXPECTED
   end
 
   def test_pack_garbage
-    assert_warn(%r%unknown pack directive '\*' in '\*U'$%) do
+    assert_raise(ArgumentError, %r%unknown pack directive '\*' in '\*U'$%) do
       assert_equal "\000", [0].pack("*U")
     end
   end
 
   def test_unpack_garbage
-    assert_warn(%r%unknown unpack directive '\*' in '\*U'$%) do
+    assert_raise(ArgumentError, %r%unknown unpack directive '\*' in '\*U'$%) do
       assert_equal [0], "\000".unpack("*U")
     end
   end
 
   def test_invalid_warning
-    assert_warning(/unknown pack directive ',' in ','/) {
+    assert_raise(ArgumentError, /unknown pack directive ',' in ','/) {
       [].pack(",")
     }
-    assert_warning(/\A[ -~]+\Z/) {
+    assert_raise(ArgumentError, /\A[ -~]+\Z/) {
       [].pack("\x7f")
     }
-    assert_warning(/\A(.* in '\u{3042}'\n)+\z/) {
+    assert_raise(ArgumentError, /\A(.* in '\u{3042}'\n)+\z/) {
       [].pack("\u{3042}")
     }
 
-    assert_warning(/\A.* in '.*U'\Z/) {
+    assert_raise(ArgumentError, /\A.* in '.*U'\Z/) {
       assert_equal "\000", [0].pack("\0U")
     }
-    assert_warning(/\A.* in '.*U'\Z/) {
+    assert_raise(ArgumentError, /\A.* in '.*U'\Z/) {
       "\000".unpack("\0U")
     }
   end