diff --git a/app/assets/javascripts/delta_input.js b/app/assets/javascripts/delta_input.js
index 0ae7f4a5..d0da30d8 100644
--- a/app/assets/javascripts/delta_input.js
+++ b/app/assets/javascripts/delta_input.js
@@ -19,7 +19,7 @@ function data_delta_update(el, direction) {
   var delta = $(el).data('delta');
   var granularity = $(el).data('granularity');
 
-  var val = $(el).val();
+  var val = $(el).val().replace(',', '.');
   var oldval = $.isNumeric(val) ? Number(val) : 0;
   var newval = oldval + delta*direction;
 
@@ -31,10 +31,11 @@ function data_delta_update(el, direction) {
   $('button[data-increment='+id+']').attr('disabled', newval>=max ? 'disabled' : null);
 
   // warn when what was entered is not a number
-  $(el).toggleClass('error', val!='' && val!='.' && (!$.isNumeric(val) || val < 0));
+  const erroneousValue = val!='' && val!='.' && (!$.isNumeric(val) || val < 0)
+  $(el).toggleClass('error', erroneousValue);
 
   // update field, unless the user is typing
-  if (!$(el).is(':focus')) {
+  if (!$(el).is(':focus') && !erroneousValue) {
     $(el).val(round_float(newval, granularity));
     $(el).trigger('changed');
   }