diff --git a/src/util.lua b/src/util.lua
index e10c21fac..b4537d6d6 100644
--- a/src/util.lua
+++ b/src/util.lua
@@ -358,15 +358,15 @@ end
 function FixUTF8(s, repl)
   local p, len, invalid = 1, #s, {}
   while p <= len do
-    if     p == s:find("[%z\1-\127]", p) then p = p + 1
-    elseif p == s:find("[\194-\223][\128-\191]", p) then p = p + 2
-    elseif p == s:find(       "\224[\160-\191][\128-\191]", p)
-        or p == s:find("[\225-\236][\128-\191][\128-\191]", p)
-        or p == s:find(       "\237[\128-\159][\128-\191]", p)
-        or p == s:find("[\238-\239][\128-\191][\128-\191]", p) then p = p + 3
-    elseif p == s:find(       "\240[\144-\191][\128-\191][\128-\191]", p)
-        or p == s:find("[\241-\243][\128-\191][\128-\191][\128-\191]", p)
-        or p == s:find(       "\244[\128-\143][\128-\191][\128-\191]", p) then p = p + 4
+    if     s:find("^[%z\1-\127]", p) then p = p + 1
+    elseif s:find("^[\194-\223][\128-\191]", p) then p = p + 2
+    elseif s:find(       "^\224[\160-\191][\128-\191]", p)
+        or s:find("^[\225-\236][\128-\191][\128-\191]", p)
+        or s:find(       "^\237[\128-\159][\128-\191]", p)
+        or s:find("^[\238-\239][\128-\191][\128-\191]", p) then p = p + 3
+    elseif s:find(       "^\240[\144-\191][\128-\191][\128-\191]", p)
+        or s:find("^[\241-\243][\128-\191][\128-\191][\128-\191]", p)
+        or s:find(       "^\244[\128-\143][\128-\191][\128-\191]", p) then p = p + 4
     else
       local repl = type(repl) == 'function' and repl(s:sub(p,p)) or repl
       s = s:sub(1, p-1)..repl..s:sub(p+1)
diff --git a/t/9-misc.lua b/t/9-misc.lua
index 3bd00fc01..b8d484269 100644
--- a/t/9-misc.lua
+++ b/t/9-misc.lua
@@ -1,3 +1,18 @@
+local fixed, invalid = FixUTF8("+\128\129\130+\194\127+", "+")
+is(fixed, "++++++\127+", "Invalid UTF8 is fixed (1/2).")
+is(#invalid, 4, "Invalid UTF8 is fixed (2/2).")
+
+local UTF8s = {
+  "ABCDE", -- 1 byte codes
+  "\194\160\194\161\194\162\194\163\194\164", -- 2 byte codes
+  "\225\160\160\225\161\161\225\162\162\225\163\163\225\164\164", -- 3 byte codes
+}
+
+for n, code in ipairs(UTF8s) do
+  is(FixUTF8(code), code, ("Valid UTF8 code is left unmodified (%d/%d)."):format(n, #UTF8s))
+end
+
+
 local editor = NewFile()
 
 for _, tst in ipairs({