diff --git a/lib/paco/index.rb b/lib/paco/index.rb
index 93555dd..99bde6d 100644
--- a/lib/paco/index.rb
+++ b/lib/paco/index.rb
@@ -4,7 +4,7 @@ module Paco
     # @param [Integer] pos
     def self.calculate(input:, pos:)
       raise ArgumentError, "`pos` must be a non-negative integer" if pos < 0
-      raise ArgumentError, "`pos` is grater then input length" if pos > input.length
+      raise ArgumentError, "`pos` is greater then input length" if pos > input.length
 
       lines = input[0..pos].lines
       line = lines.empty? ? 1 : lines.length
diff --git a/spec/paco/index_spec.rb b/spec/paco/index_spec.rb
index 0a07f63..1f0560b 100644
--- a/spec/paco/index_spec.rb
+++ b/spec/paco/index_spec.rb
@@ -17,10 +17,16 @@
   end
 
   it "raises an error when pos < 0" do
-    expect { index.calculate(input: input, pos: -1) }.to raise_error(ArgumentError)
+    expect { index.calculate(input: input, pos: -1) }.to raise_error(
+      ArgumentError,
+      "`pos` must be a non-negative integer"
+    )
   end
 
   it "raises an error when pos > input length" do
-    expect { index.calculate(input: input, pos: 1000) }.to raise_error(ArgumentError)
+    expect { index.calculate(input: input, pos: 1000) }.to raise_error(
+      ArgumentError,
+      "`pos` is greater then input length"
+    )
   end
 end