diff --git a/src/nano.c b/src/nano.c
index 307e7bd5990f13685f38b3c64abd155d0230715b..e9035a5eb0f84758a233dfe9de411d99bcc7b937 100644
--- a/src/nano.c
+++ b/src/nano.c
@@ -2643,7 +2643,7 @@ void justify_format(filestruct *paragraph, size_t skip)
     assert(paragraph != NULL);
     assert(paragraph->data != NULL);
     assert(skip < strlen(paragraph->data));
-    assert(!isblank(paragraph->data[skip]));
+    assert(!is_blank_mbchar(paragraph->data + skip));
 
     end = paragraph->data + skip;
     new_paragraph_data = charalloc(strlen(paragraph->data) + 1);