diff --git a/gio/gdatainputstream.c b/gio/gdatainputstream.c
index 0a1672409..fa3842d8d 100644
--- a/gio/gdatainputstream.c
+++ b/gio/gdatainputstream.c
@@ -840,7 +840,11 @@ g_data_input_stream_read_line_utf8 (GDataInputStream  *stream,
       g_set_error_literal (error, G_CONVERT_ERROR,
 			   G_CONVERT_ERROR_ILLEGAL_SEQUENCE,
 			   _("Invalid byte sequence in conversion input"));
+
+      if (length != NULL)
+        *length = 0;
       g_free (res);
+
       return NULL;
     }
   return res;
diff --git a/gio/tests/data-input-stream.c b/gio/tests/data-input-stream.c
index e3f131f68..55fc304e4 100644
--- a/gio/tests/data-input-stream.c
+++ b/gio/tests/data-input-stream.c
@@ -174,8 +174,17 @@ test_read_lines_LF_valid_utf8 (void)
       gsize length = -1;
       line = g_data_input_stream_read_line_utf8 (G_DATA_INPUT_STREAM (stream), &length, NULL, &error);
       g_assert_no_error (error);
+
       if (line == NULL)
-	break;
+        {
+          g_assert_cmpuint (length, ==, 0);
+          break;
+        }
+      else
+        {
+          g_assert_cmpuint (length, >, 0);
+        }
+
       n_lines++;
       g_free (line);
     }
@@ -207,11 +216,16 @@ test_read_lines_LF_invalid_utf8 (void)
       gsize length = -1;
       line = g_data_input_stream_read_line_utf8 (G_DATA_INPUT_STREAM (stream), &length, NULL, &error);
       if (n_lines == 0)
-	g_assert_no_error (error);
+        {
+          /* First line is valid UTF-8 */
+          g_assert_no_error (error);
+          g_assert_cmpuint (length, ==, 3);
+        }
       else
 	{
           g_assert_error (error, G_CONVERT_ERROR, G_CONVERT_ERROR_ILLEGAL_SEQUENCE);
 	  g_clear_error (&error);
+          g_assert_cmpuint (length, ==, 0);
 	  g_free (line);
 	  break;
 	}