diff --git a/vvp/vpi_callback.cc b/vvp/vpi_callback.cc
index 89917d555..5351dc1f4 100644
--- a/vvp/vpi_callback.cc
+++ b/vvp/vpi_callback.cc
@@ -881,7 +881,7 @@ static void real_signal_value(struct t_vpi_value*vp, double rval)
 	    break;
 
 	  case vpiDecStrVal:
-	    if (isnan(rval))
+	    if (std::isnan(rval))
 		  sprintf(rbuf, "%s", "nan");
 	    else
 		  sprintf(rbuf, "%0.0f", vlg_round(rval));
diff --git a/vvp/vpi_vthr_vector.cc b/vvp/vpi_vthr_vector.cc
index 69919fa30..3ca433337 100644
--- a/vvp/vpi_vthr_vector.cc
+++ b/vvp/vpi_vthr_vector.cc
@@ -132,7 +132,7 @@ static void vthr_real_get_value(vpiHandle ref, s_vpi_value*vp)
 	    break;
 
 	  case vpiDecStrVal:
-	    if (isnan(val))
+	    if (std::isnan(val))
 		  sprintf(rbuf, "%s", "nan");
 	    else
 		  sprintf(rbuf, "%0.0f", vlg_round(val));