diff --git a/lib/time_range/time.rb b/lib/time_range/time.rb
index ec73246..5c3b546 100644
--- a/lib/time_range/time.rb
+++ b/lib/time_range/time.rb
@@ -15,6 +15,8 @@ def initialize(time, by = nil)
     end
 
     def succ
+      raise ArgumentError, 'by() not set for this TimeRange' if !@by || @by.empty?
+
       next_time = Advancer.advance_time(@time, @by)
       return self.class.new(next_time, @by)
     end
diff --git a/spec/time_range_spec.rb b/spec/time_range_spec.rb
index bf15579..d335844 100644
--- a/spec/time_range_spec.rb
+++ b/spec/time_range_spec.rb
@@ -92,6 +92,13 @@
   end
 
   shared_examples 'it has a begin' do
+    it 'raises gracefully if #by is not called' do
+      range = described_class.new(first, last)
+      expect {
+        range.first(1)
+      }.to raise_error ArgumentError, /by\(\) not set for this TimeRange/
+    end
+
     describe '#begin' do
       it 'has a begin' do
         expect(range.begin).to eq first