diff --git a/lib/pure/strutils.nim b/lib/pure/strutils.nim
index cc55301752..e373baa46d 100644
--- a/lib/pure/strutils.nim
+++ b/lib/pure/strutils.nim
@@ -435,7 +435,7 @@ proc isNilOrWhitespace*(s: string): bool {.noSideEffect, procvar, rtl, extern: "
 proc substrEq(s: string, pos: int, substr: string): bool =
   var i = 0
   var length = substr.len
-  while i < length and s[pos+i] == substr[i]:
+  while i < length and pos+i < s.len and s[pos+i] == substr[i]:
     inc i
   return i == length