diff --git a/attributes/maestro-schema.c b/attributes/maestro-schema.c
index d08fe67b7095cf03bfb267215d1e911b40864504..ac87c0d3e79c38504f2614a78575c289afd29493 100644
--- a/attributes/maestro-schema.c
+++ b/attributes/maestro-schema.c
@@ -730,10 +730,12 @@ mstro_attribute_val__compute_size(enum mstro_stp_val_kind kind,
       *val_size = sizeof(double); break;
     case MSTRO_STP_STR:
     case MSTRO_STP_REGEX: 
-      if(string)
+      if(string!=NULL)
         *val_size = strlen(string)+1;
-      else
+      else {
+	assert(val!=NULL);
         *val_size = strlen((char*)val)+1;
+      }
       break;
     case MSTRO_STP_TIMESTAMP:
       *val_size = sizeof(mstro_timestamp);