diff --git a/gio/gdatainputstream.c b/gio/gdatainputstream.c
index ce6175951..ef728f006 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 8a1e8fb28..11c997bce 100644
--- a/gio/tests/data-input-stream.c
+++ b/gio/tests/data-input-stream.c
@@ -80,9 +80,9 @@ test_read_lines (GDataStreamNewlineType newline_type)
     lines[i] = "some_text";
 	
   base_stream = g_memory_input_stream_new ();
-  g_assert (base_stream != NULL);
+  g_assert_nonnull (base_stream);
   stream = G_INPUT_STREAM (g_data_input_stream_new (base_stream));
-  g_assert(stream != NULL);
+  g_assert_nonnull (stream);
 	
   /*  Byte order testing */
   g_data_input_stream_set_byte_order (G_DATA_INPUT_STREAM (stream), G_DATA_STREAM_BYTE_ORDER_BIG_ENDIAN);
@@ -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 != NULL);
+          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;
 	}
@@ -354,7 +368,7 @@ test_read_upto (void)
               line++;
 
               stop_char = g_data_input_stream_read_byte (G_DATA_INPUT_STREAM (stream), NULL, &error);
-              g_assert (memchr (DATA_SEP, stop_char, DATA_SEP_LEN) != NULL);
+              g_assert_nonnull (memchr (DATA_SEP, stop_char, DATA_SEP_LEN));
               g_assert_no_error (error);
             }
           g_free (data);