diff --git a/lib/common.c b/lib/common.c
index d1b028fe..deb94eec 100644
--- a/lib/common.c
+++ b/lib/common.c
@@ -195,7 +195,7 @@ int _sasl_add_string(char **out, size_t *alloclen,
     return SASL_NOMEM;
 
   strncpy(*out + *outlen, add, addlen);
-  *outlen += addlen;
+  *outlen += addlen-1;
 
   return SASL_OK;
 }