diff --git a/csrc/pf_float.h b/csrc/pf_float.h
index d2b1791..6539efc 100644
--- a/csrc/pf_float.h
+++ b/csrc/pf_float.h
@@ -36,6 +36,7 @@ typedef double PF_FLOAT;
 #define fp_floor  floor
 #define fp_log    log
 #define fp_log10  log10
+#define fp_log1p  log1p
 #define fp_pow    pow
 #define fp_sin    sin
 #define fp_sinh   sinh
diff --git a/csrc/pfinnrfp.h b/csrc/pfinnrfp.h
index 4a3095b..3e35185 100644
--- a/csrc/pfinnrfp.h
+++ b/csrc/pfinnrfp.h
@@ -301,7 +301,7 @@
         break;
 
     case ID_FP_FLNP1: /* ( -- ) ( F: r1 -- r2 ) */
-        FP_TOS = (PF_FLOAT) (fp_log(FP_TOS) + 1.0);
+        FP_TOS = (PF_FLOAT) fp_log1p(FP_TOS); /* log(x+1) */
         break;
 
     case ID_FP_FLOG: /* ( -- ) ( F: r1 -- r2 ) */