diff --git a/docs/reference.md b/docs/reference.md
index 4a78518..366686d 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -322,6 +322,7 @@ Public error classes:
 - `FixtureKit::FixtureDefinitionNotFound`
 - `FixtureKit::RunnerAlreadyStartedError`
 - `FixtureKit::CircularFixtureInheritance`
+- `FixtureKit::MissingCoderDataError`
 
 ## Requirements
 
diff --git a/lib/fixture_kit.rb b/lib/fixture_kit.rb
index 7caf094..e6458f7 100644
--- a/lib/fixture_kit.rb
+++ b/lib/fixture_kit.rb
@@ -9,6 +9,7 @@ class CacheMissingError < Error; end
   class FixtureDefinitionNotFound < Error; end
   class RunnerAlreadyStartedError < Error; end
   class CircularFixtureInheritance < Error; end
+  class MissingCoderDataError < Error; end
 
   autoload :VERSION, File.expand_path("fixture_kit/version", __dir__)
   autoload :Configuration, File.expand_path("fixture_kit/configuration", __dir__)
diff --git a/lib/fixture_kit/memory_cache.rb b/lib/fixture_kit/memory_cache.rb
index dd9aa1c..61f166e 100644
--- a/lib/fixture_kit/memory_cache.rb
+++ b/lib/fixture_kit/memory_cache.rb
@@ -11,7 +11,13 @@ def initialize(data:, exposed:)
     end
 
     def data_for(coder_class)
-      data.fetch(coder_class)
+      data.fetch(coder_class) do
+        raise MissingCoderDataError,
+          "The fixture cache has no data for #{coder_class}. This usually means " \
+          "the cache was written before #{coder_class} was registered as a coder. " \
+          "Regenerate the fixture cache so it includes this coder (clear the cache " \
+          "directory, or run without FIXTURE_KIT_PRESERVE_CACHE=1)."
+      end
     end
   end
 end
diff --git a/spec/unit/memory_cache_spec.rb b/spec/unit/memory_cache_spec.rb
index 3fb916a..4a1a4b7 100644
--- a/spec/unit/memory_cache_spec.rb
+++ b/spec/unit/memory_cache_spec.rb
@@ -20,4 +20,28 @@
       expect(cache).to be_frozen
     end
   end
+
+  describe "#data_for" do
+    it "returns the data stored for a coder class" do
+      cache = described_class.new(
+        data: { FixtureKit::ActiveRecordCoder => { "User" => "SQL" } },
+        exposed: {}
+      )
+
+      expect(cache.data_for(FixtureKit::ActiveRecordCoder)).to eq({ "User" => "SQL" })
+    end
+
+    it "raises an actionable error when the cache predates a coder" do
+      # A cache written before this coder was registered has no entry for it.
+      # Rather than an opaque KeyError, callers should learn the cache is stale
+      # and how to fix it — naming the coder and pointing to regeneration.
+      cache = described_class.new(data: {}, exposed: {})
+
+      expect { cache.data_for(FixtureKit::ActiveRecordCoder) }
+        .to raise_error(FixtureKit::MissingCoderDataError) do |error|
+          expect(error.message).to include("FixtureKit::ActiveRecordCoder")
+          expect(error.message).to match(/regenerate/i)
+        end
+    end
+  end
 end