diff --git a/tools/nimgrep.nim b/tools/nimgrep.nim
index 8c6353e319..b26d862b6a 100644
--- a/tools/nimgrep.nim
+++ b/tools/nimgrep.nim
@@ -160,7 +160,7 @@ proc processFile(pattern; filename: string; counter: var int) =
   var reallyReplace = true
   while i < buffer.len:
     let t = findBounds(buffer, pattern, matches, i)
-    if t.first < 0: break
+    if t.first < 0 or t.last < t.first: break
     inc(line, countLines(buffer, i, t.first-1))
 
     var wholeMatch = buffer.substr(t.first, t.last)