--- src/rrd_tool.c
+++ src/rrd_tool.c
@@ -724,9 +724,9 @@
 	    for (j = 0; j < col_cnt; j++) {
 	      rrd_value_t newval = DNAN;
               if (enumds == 1)
-		snprintf(vtag,15,"%s%lu", COL_DATA_TAG, j);
+		snprintf(vtag,sizeof(vtag),"%s%lu", COL_DATA_TAG, j);
 	      else
-		snprintf(vtag,15,"%s",COL_DATA_TAG);
+		snprintf(vtag,sizeof(vtag),"%s",COL_DATA_TAG);
               
 	      newval = *ptr;
 	      if(isnan(newval)){