diff --git a/spec/db_spec.cr b/spec/db_spec.cr
index ec42f30..c619c9d 100644
--- a/spec/db_spec.cr
+++ b/spec/db_spec.cr
@@ -107,5 +107,9 @@ describe DB do
 
     DB.fetch_bool(HTTP::Params.parse("bar=true"), "foo", false).should be_false
     DB.fetch_bool(HTTP::Params.parse("bar=true"), "foo", true).should be_true
+
+    expect_raises(ArgumentError, %(invalid "other" value for option "foo")) do
+      DB.fetch_bool(HTTP::Params.parse("foo=other"), "foo", true)
+    end
   end
 end
diff --git a/src/db.cr b/src/db.cr
index 43a90f4..74e3d98 100644
--- a/src/db.cr
+++ b/src/db.cr
@@ -123,10 +123,15 @@ module DB
 
   # :nodoc:
   def self.fetch_bool(params : HTTP::Params, name, default : Bool)
-    if value = params[name]?
-      value.underscore == "true"
-    else
+    case (value = params[name]?).try &.downcase
+    when nil
       default
+    when "true"
+      true
+    when "false"
+      false
+    else
+      raise ArgumentError.new(%(invalid "#{value}" value for option "#{name}"))
     end
   end
 end