diff --git a/lib/guard/listener.rb b/lib/guard/listener.rb
index 9ea5948..1017c51 100644
--- a/lib/guard/listener.rb
+++ b/lib/guard/listener.rb
@@ -1,4 +1,5 @@
 require 'rbconfig'
+require 'digest/sha1'
 
 module Guard
 
@@ -53,7 +54,7 @@ module Guard
     end
 
     def file_content_modified?(path)
-      sha1_checksum = Digest::SHA1.file(path).to_s
+      sha1_checksum = ::Digest::SHA1.file(path).to_s
       if sha1_checksums_hash[path] != sha1_checksum
         @sha1_checksums_hash[path] = sha1_checksum
         true
diff --git a/lib/guard/watcher.rb b/lib/guard/watcher.rb
index fc16b23..074e7c2 100644
--- a/lib/guard/watcher.rb
+++ b/lib/guard/watcher.rb
@@ -59,4 +59,4 @@ module Guard
     end
     
   end
-end
\ No newline at end of file
+end