diff --git a/lib/sirius/time_converter.rb b/lib/sirius/time_converter.rb
index b4b94e2..78938b9 100644
--- a/lib/sirius/time_converter.rb
+++ b/lib/sirius/time_converter.rb
@@ -32,7 +32,9 @@ def initialize( hour_starts:, hour_length:)
     # @return [Period] Event start and end times
     #
     def convert_time( start_hour, duration )
-      raise "Invalid start hour (#{start_hour}) or duration (#{duration})" if start_hour <= 0 || duration <= 0
+      if start_hour <= 0 || duration <= 0 || !@hour_starts.key?(start_hour - 1)
+        raise "Invalid start hour (#{start_hour}) or duration (#{duration})"
+      end
       start_time = @hour_starts[start_hour - 1]
       end_time = start_time + (@hour_length * duration).minutes
       Period.new(start_time, end_time)
diff --git a/spec/lib/sirius/time_converter_spec.rb b/spec/lib/sirius/time_converter_spec.rb
index e187cb8..ee33b6e 100644
--- a/spec/lib/sirius/time_converter_spec.rb
+++ b/spec/lib/sirius/time_converter_spec.rb
@@ -22,10 +22,14 @@
     expect(result).to eq(Period.parse('18:00','20:15'))
   end
 
-  it 'raises error with invalid start time' do
+  it 'raises error with zero start_hour' do
     expect { converter.convert_time(0, 2) }.to raise_error(RuntimeError)
   end
 
+  it 'raises error with unknown start_hour' do
+    expect { converter.convert_time(66, 2) }.to raise_error(RuntimeError)
+  end
+
   it 'raises error with zero duration' do
     expect { converter.convert_time(5, 0) }.to raise_error(RuntimeError)
   end