In libguile/numbers.c abs() is used for long argument where labs() should be used.  
A patch is included below for guile-2.1.7.

/* Returns log(n/d), for exact non-zero integers n and d */
static SCM
log_of_fraction (SCM n, SCM d)
{
  long n_size = scm_to_long (scm_integer_length (n));
  long d_size = scm_to_long (scm_integer_length (d));

  if (abs (n_size - d_size) > 1)
    return (scm_difference (log_of_exact_integer (n),
                            log_of_exact_integer (d)));


--- libguile/numbers.c.orig	2017-02-18 08:29:52.000000000 -0800
+++ libguile/numbers.c	2017-02-18 08:30:23.000000000 -0800
@@ -9951,7 +9951,7 @@
   long n_size = scm_to_long (scm_integer_length (n));
   long d_size = scm_to_long (scm_integer_length (d));
 
-  if (abs (n_size - d_size) > 1)
+  if (labs (n_size - d_size) > 1)
     return (scm_difference (log_of_exact_integer (n),
 			    log_of_exact_integer (d)));
   else if (scm_is_false (scm_negative_p (n)))