diff --git a/autoload/gitgutter/diff_highlight.vim b/autoload/gitgutter/diff_highlight.vim
index 7bd48e0..c2c48e1 100644
--- a/autoload/gitgutter/diff_highlight.vim
+++ b/autoload/gitgutter/diff_highlight.vim
@@ -162,6 +162,9 @@ endif
 "
 function! s:common_prefix(a, b)
   let len = min([len(a:a), len(a:b)])
+  if len == 0
+    return -1
+  endif
   for i in range(len)
     if a:a[i:i] != a:b[i:i]
       return i - 1
diff --git a/test/test_gitgutter.vim b/test/test_gitgutter.vim
index a65e0f1..20044bb 100644
--- a/test/test_gitgutter.vim
+++ b/test/test_gitgutter.vim
@@ -916,6 +916,9 @@ endfunction
 
 
 function Test_common_prefix()
+  " zero length
+  call assert_equal(-1, gitgutter#diff_highlight#common_prefix('', 'foo'))
+  call assert_equal(-1, gitgutter#diff_highlight#common_prefix('foo', ''))
   " nothing in common
   call assert_equal(-1, gitgutter#diff_highlight#common_prefix('-abcde', '+pqrst'))
   call assert_equal(-1, gitgutter#diff_highlight#common_prefix('abcde', 'pqrst'))