diff --git a/src/fileio.c b/src/fileio.c
index 81a7b50..eb46f1f 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -2917,15 +2917,16 @@
 	{
 	    int header_len;
 
-	    curbuf->b_cryptstate = crypt_create_from_header(
-						       method, cryptkey, ptr);
-	    crypt_set_cm_option(curbuf, method);
-
-	    // Remove cryptmethod specific header from the text.
 	    header_len = crypt_get_header_len(method);
 	    if (*sizep <= header_len)
 		// invalid header, buffer can't be encrypted
 		return NULL;
+
+	    curbuf->b_cryptstate = crypt_create_from_header(
+							method, cryptkey, ptr);
+	    crypt_set_cm_option(curbuf, method);
+
+	    // Remove cryptmethod specific header from the text.
 	    *filesizep += header_len;
 	    *sizep -= header_len;
 	    mch_memmove(ptr, ptr + header_len, (size_t)*sizep);