diff --git a/exe/deprecations b/exe/deprecations
index db824d1..aff4176 100755
--- a/exe/deprecations
+++ b/exe/deprecations
@@ -3,10 +3,14 @@ require "json"
 require "rainbow"
 require "optparse"
 require "set"
+require_relative "../lib/deprecation_tracker/valid_modes"
 require_relative "../lib/deprecation_tracker/shard_merger"
 
 def run_tests(deprecation_warnings, opts = {})
   tracker_mode = opts[:tracker_mode]
+  if tracker_mode && !DeprecationTracker::VALID_MODES.include?(tracker_mode.to_sym)
+    abort "Invalid --tracker-mode #{tracker_mode.inspect}. Must be one of: #{DeprecationTracker::VALID_MODES_DISPLAY}."
+  end
   next_mode = opts[:next_mode]
   rspec_command = if next_mode
     "bin/next rspec"
@@ -69,7 +73,7 @@ option_parser = OptionParser.new do |opts|
     options[:next] = next_mode
   end
 
-  opts.on("--tracker-mode MODE", "Set DEPRECATION_TRACKER in test mode. Options: save or compare") do |tracker_mode|
+  opts.on("--tracker-mode MODE", "Set DEPRECATION_TRACKER in test mode. Options: #{DeprecationTracker::VALID_MODES_DISPLAY}") do |tracker_mode|
     options[:tracker_mode] = tracker_mode
   end
 
diff --git a/lib/deprecation_tracker.rb b/lib/deprecation_tracker.rb
index 2c357be..f83ad27 100644
--- a/lib/deprecation_tracker.rb
+++ b/lib/deprecation_tracker.rb
@@ -10,6 +10,8 @@
 # Tracks deprecation warnings, grouped by spec file. After the test run, compare against shitlist of expected
 # deprecation warnings. If anything is added or removed, raise an error with a diff of the changes.
 #
+require_relative "deprecation_tracker/valid_modes"
+
 class DeprecationTracker
   UnexpectedDeprecations = Class.new(StandardError)
 
@@ -141,6 +143,9 @@ def initialize(shitlist_path, transform_message = nil, mode = :save, node_index:
     @transform_message = transform_message || -> (message) { message }
     @deprecation_messages = {}
     @mode = mode ? mode.to_sym : :save
+    unless VALID_MODES.include?(@mode)
+      raise ArgumentError, "mode must be one of #{VALID_MODES_DISPLAY}, got: #{mode.inspect}"
+    end
     if @mode == :compare && node_index
       raise ArgumentError, "node_index cannot be used with compare mode"
     end
diff --git a/lib/deprecation_tracker/valid_modes.rb b/lib/deprecation_tracker/valid_modes.rb
new file mode 100644
index 0000000..7afe503
--- /dev/null
+++ b/lib/deprecation_tracker/valid_modes.rb
@@ -0,0 +1,4 @@
+class DeprecationTracker
+  VALID_MODES = %i[save compare].freeze
+  VALID_MODES_DISPLAY = VALID_MODES.map(&:to_s).join(", ").freeze
+end
diff --git a/spec/deprecation_tracker_spec.rb b/spec/deprecation_tracker_spec.rb
index 67bb311..9e0064c 100644
--- a/spec/deprecation_tracker_spec.rb
+++ b/spec/deprecation_tracker_spec.rb
@@ -264,11 +264,10 @@
       expect(tracker.mode).to eq(:save)
     end
 
-    it "does not save nor compare if mode is invalid" do
-      tracker = DeprecationTracker.new(shitlist_path, nil, "random_stuff")
-      expect(tracker).not_to receive(:save)
-      expect(tracker).not_to receive(:compare)
-      tracker.after_run
+    it "raises ArgumentError for invalid mode" do
+      expect {
+        DeprecationTracker.new(shitlist_path, nil, "random_stuff")
+      }.to raise_error(ArgumentError)
     end
   end
 
@@ -426,6 +425,27 @@ def self.behavior
       tracker = DeprecationTracker.init_tracker({})
       expect(tracker.mode).to eq(:compare)
     end
+
+    it "raises ArgumentError when ENV['DEPRECATION_TRACKER'] is invalid" do
+      stub_const("ENV", ENV.to_h.merge("DEPRECATION_TRACKER" => "bogus"))
+      expect {
+        DeprecationTracker.init_tracker({})
+      }.to raise_error(ArgumentError)
+    end
+  end
+
+  describe "VALID_MODES" do
+    it "contains save and compare" do
+      expect(DeprecationTracker::VALID_MODES).to include(:save, :compare)
+    end
+
+    it "accepts all valid modes without error" do
+      DeprecationTracker::VALID_MODES.each do |mode|
+        expect {
+          DeprecationTracker.new(shitlist_path, nil, mode)
+        }.not_to raise_error
+      end
+    end
   end
 
   describe DeprecationTracker::KernelWarnTracker do