diff --git a/lib/erb.rb b/lib/erb.rb
index c9b987dfb7..d8a91e34bd 100644
--- a/lib/erb.rb
+++ b/lib/erb.rb
@@ -668,6 +668,8 @@ def prepare_trim_mode(mode) # :nodoc:
       when 0
         return [false, nil]
       when String
+        warn "Invalid ERB trim mode: #{mode.inspect}", uplevel: 5 unless mode.match?(/\A(%|-|>|<>){1,2}\z/)
+
         perc = mode.include?('%')
         if mode.include?('-')
           return [perc, '-']
@@ -679,6 +681,7 @@ def prepare_trim_mode(mode) # :nodoc:
           [perc, nil]
         end
       else
+        warn "Invalid ERB trim mode: #{mode.inspect}", uplevel: 5 unless mode.nil?
         return [false, nil]
       end
     end
diff --git a/test/erb/test_erb.rb b/test/erb/test_erb.rb
index ffa1fef23d..8da5051579 100644
--- a/test/erb/test_erb.rb
+++ b/test/erb/test_erb.rb
@@ -235,6 +235,24 @@ def test_explicit_trim_line_with_carriage_return
     assert_equal("line\r\n" * 3, erb.result)
   end
 
+  def test_invalid_trim_mode
+    assert_warning(/Invalid ERB trim mode/) do
+      @erb.new("", trim_mode: 'abc-def')
+    end
+
+    assert_warning(/Invalid ERB trim mode/) do
+      @erb.new("", trim_mode: '%<')
+    end
+
+    assert_warning(/Invalid ERB trim mode/) do
+      @erb.new("", trim_mode: '%<>-')
+    end
+
+    assert_warning(/Invalid ERB trim mode/) do
+      @erb.new("", trim_mode: 3)
+    end
+  end
+
   def test_run
     out = StringIO.new
     orig, $stdout = $stdout, out