diff --git a/src/textformat.c b/src/textformat.c
index 6a93890..7ebbc88 100644
--- a/src/textformat.c
+++ b/src/textformat.c
@@ -540,6 +540,9 @@
     if (leader1_len == 0)
 	return (leader2_len == 0);
 
+    char_u  *lnum_line = NULL;
+    int	    line_len = 0;
+
     // If first leader has 'f' flag, the lines can be joined only if the
     // second line does not have a leader.
     // If first leader has 'e' flag, the lines can never be joined.
@@ -555,7 +558,12 @@
 		return FALSE;
 	    if (*p == COM_START)
 	    {
-		if (*(ml_get(lnum) + leader1_len) == NUL)
+		if (lnum_line == NULL)
+		{
+		    lnum_line = ml_get(lnum);
+		    line_len = (int)STRLEN(lnum_line);
+		}
+		if (line_len <= leader1_len)
 		    return FALSE;
 		if (leader2_flags == NULL || leader2_len == 0)
 		    return FALSE;