dovecot 2.3.13 : make check FAILURE : Assert failed: buffer_append_full_file

[J Lumby]

After successfully building dovecot 2.3.13 (linux kernel 5.10.1 , 
glibc-2.32 , gcc-9.3)

make check had one failure as follows :

test-buffer-istream.c:54: Assert failed: buffer_append_full_file(result, 
TEST_FILENAME, SIZE_MAX, &error) == BUFFER_APPEND_READ_ERROR
test-buffer-istream.c:56: Assert failed: error != NULL && *error != '\0'
buffer_append_full_file .............................................. : 
FAILED

After a brief look at the referenced code in test-buffer-istream.c  ,  I 
can't find out what this FAIL really means or whether it is serious.

The assert is finding that the result of the call to 
buffer_append_full_file is not BUFFER_APPEND_READ_ERROR,   but it does 
not report what the result code actually was.    Maybe errno was 
actually 0 and call succeeded?   I don't know.   Nor can I see how to 
add some code to find out.

Can anyone offer any advice on what this means and/or how to investigate?

Cheers,  John Lumby