diff --git a/spec/nextfile_spec.lua b/spec/nextfile_spec.lua
index 7b4e430..7a2542e 100644
--- a/spec/nextfile_spec.lua
+++ b/spec/nextfile_spec.lua
@@ -68,12 +68,20 @@ describe("nextfile", function()
 	end)
 
 	insulate("prevfile.prevfile returns a non-numeric filename", function()
-		prevfile.prevfile = function() return "/testdir/421b.txt" end
+		it("should return the result of replacing the last character with the next letter of the alphabet", function()
+			prevfile.prevfile = function() return "/testdir/421b.txt" end
 
-		--it("should return the result of replacing the last character with the next letter of the alphabet", function()
-			--local file = nextfile.nextfile("/testdir")
+			local file = nextfile.nextfile("/testdir")
 
-			--assert.are.equal("/testdir/421c.txt", file)
-		--end)
+			assert.are.equal("/testdir/421c.txt", file)
+		end)
+
+		it("should return the result of replacing z with za when z is the last letter of the filename", function()
+			prevfile.prevfile = function() return "/testdir/421z.txt" end
+
+			local file = nextfile.nextfile("/testdir")
+
+			assert.are.equal("/testdir/421za.txt", file)
+		end)
 	end)
 end)
diff --git a/src/nextfile.lua b/src/nextfile.lua
index e9ada7f..da63c6c 100644
--- a/src/nextfile.lua
+++ b/src/nextfile.lua
@@ -23,7 +23,8 @@ function nextfile.nextfile(directory)
 
 	if tonumber(name) == nil then
 		local lastletter = name:sub(#name)
-		return dir .. name:gsub() .. ext
+		local nextletter = nextfile.nextletter(lastletter)
+		return dir .. name:gsub(lastletter .. "$", nextletter) .. ext
 	end
 	
 	return dir .. name + 1 .. ext