diff --git a/src/hunspell/affixmgr.cxx b/src/hunspell/affixmgr.cxx
index 96262eb..cc09f6a 100644
--- a/src/hunspell/affixmgr.cxx
+++ b/src/hunspell/affixmgr.cxx
@@ -1088,7 +1088,7 @@
 // return 1 if s1 is a leading subset of s2 (dots are for infixes)
 inline int AffixMgr::isSubset(const char * s1, const char * s2)
  {
-    while (((*s1 == *s2) || (*s1 == '.')) && (*s1 != '\0')) {
+    while (((*s1 == *s2) || (*s1 == '.')) && (*s1 != '\0') && (*s2 != '\0')) {
         s1++;
         s2++;
     }