diff --git a/girepository/girnode-private.h b/girepository/girnode-private.h
index 30fbab6cc..353d6d6ca 100644
--- a/girepository/girnode-private.h
+++ b/girepository/girnode-private.h
@@ -384,7 +384,7 @@ struct _GIIrNodeUnion
   size_t size;
   GIIrOffsetsState offsets_state;
 
-  int discriminator_offset;
+  size_t discriminator_offset;
   GIIrNodeType *discriminator_type;  /* (owned) */
 };
 
diff --git a/girepository/girparser.c b/girepository/girparser.c
index 070f591bd..0d65590f6 100644
--- a/girepository/girparser.c
+++ b/girepository/girparser.c
@@ -2810,6 +2810,8 @@ start_discriminator (GMarkupParseContext  *context,
 {
   const char *type;
   const char *offset;
+  guint64 parsed_offset;
+
   if (!(strcmp (element_name, "discriminator") == 0 &&
         ctx->state == STATE_UNION))
     return FALSE;
@@ -2829,8 +2831,11 @@ start_discriminator (GMarkupParseContext  *context,
 
   ((GIIrNodeUnion *)CURRENT_NODE (ctx))->discriminator_type
     = parse_type (ctx, type);
-  ((GIIrNodeUnion *)CURRENT_NODE (ctx))->discriminator_offset
-    = atoi (offset);
+
+  if (g_ascii_string_to_unsigned (offset, 10, 0, G_MAXSIZE, &parsed_offset, error))
+    ((GIIrNodeUnion *)CURRENT_NODE (ctx))->discriminator_offset = parsed_offset;
+  else
+    return FALSE;
 
   return TRUE;
 }