diff --git a/string_pool.cc b/string_pool.cc
index a4e3e8bf5..0ab23758b 100644
--- a/string_pool.cc
+++ b/string_pool.cc
@@ -125,7 +125,7 @@ const char *string_pool_substr(const char *s, const char *t)
 }
 
 const char *string_pool_substr_strip(const char *s, const char *t) {
-  while (isspace((int) (unsigned char) *s))
+  while (s < t && isspace((int) (unsigned char) *s))
     s++;
   while (t > s && isspace((int) (unsigned char) *(t - 1)))
     t--;