diff --git a/spec/dummy_driver_spec.cr b/spec/dummy_driver_spec.cr
index 688a71d..41f4d10 100644
--- a/spec/dummy_driver_spec.cr
+++ b/spec/dummy_driver_spec.cr
@@ -246,6 +246,8 @@ describe DummyDriver do
               rs.read(Int64, Int64).should eq({3i64, 4i64})
             when 1
               rs.read(Int64, Int64).should eq({1i64, 2i64})
+            else
+              raise "unreachable"
             end
             i += 1
           end
diff --git a/src/spec.cr b/src/spec.cr
index b8c64c2..225a419 100644
--- a/src/spec.cr
+++ b/src/spec.cr
@@ -505,6 +505,8 @@ module DB
                 end
               end
             end
+          else
+            raise "Invalid prepared value. Allowed values are :both and :default"
           end
         end
       end